docs(steering): add claude-skills steering file for .claude/skills/* paths

.agents/steering/README.md

@@ -69,11 +69,13 @@ By including only relevant guidance, we reduce token usage:
 ```text
 .agents/steering/
 ├── README.md                 # This file
-├── csharp-patterns.md       # C# coding standards (Phase 4)
 ├── agent-prompts.md         # Agent prompt patterns (Phase 4)
-├── testing-approach.md      # Testing conventions (Phase 4)
+├── claude-skills.md         # Claude skill development standards
+├── csharp-patterns.md       # C# coding standards (Phase 4)
+├── documentation.md         # Documentation standards (Phase 4)
+├── powershell-patterns.md   # PowerShell coding patterns
 ├── security-practices.md    # Security requirements (Phase 4)
-└── documentation.md         # Documentation standards (Phase 4)
+└── testing-approach.md      # Testing conventions (Phase 4)
 ```
 
 ## Steering File Format
@@ -148,6 +150,7 @@ status: [placeholder | draft | published]
 | File | applyTo Pattern | Purpose | Copilot Entry Point |
 |------|-----------------|---------|---------------------|
 | `agent-prompts.md` | `**/AGENTS.md,src/claude/**/*.md,templates/agents/**/*.md` | Agent prompt standards | `.github/instructions/agent-prompts.instructions.md` |
+| `claude-skills.md` | `.claude/skills/**` | Skill development standards | `.github/instructions/claude-skills.instructions.md` |
 | `security-practices.md` | `**/Auth/**,*.env*,**/*.secrets.*,.github/workflows/**,.githooks/**` | Security best practices | `.github/instructions/security.instructions.md` |
 | `testing-approach.md` | `**/*.Tests.ps1` | Pester testing conventions | `.github/instructions/testing.instructions.md` |
 | `documentation.md` | `**/*.md` (excluding agents/steering) | Documentation standards | `.github/instructions/documentation.instructions.md` |

.agents/steering/claude-skills.md

@@ -0,0 +1,184 @@
+---
+name: Claude Skills
+applyTo: ".claude/skills/**"
+excludeFrom: ".claude/skills/**/*.Tests.ps1"
+priority: 8
+version: 1.0.0
+status: active
+---
+
+# Claude Skills Steering
+
+## Scope
+
+**Applies to**:
+
+- `.claude/skills/**` - All files in the Claude skills directory
+
+**Excludes**:
+
+- `.claude/skills/**/*.Tests.ps1` - Pester test files (see testing-approach.md)
+
+## Pre-Flight Checks
+
+Before creating or modifying a skill, verify:
+
+1. **New or update?** Check `.claude/skills/` for an existing skill with overlapping purpose.
+2. **Duplicate check**: Search skill SKILL.md files for similar `description` or `triggers`.
+3. **Memory context**: Load the skill's Serena memory if it exists (e.g., `skills-<name>-index`).
+
+## Guidelines
+
+### Skill Structure
+
+Every skill MUST have this directory layout:
+
+```text
+.claude/skills/<skill-name>/
+├── SKILL.md              # Frontmatter + prompt (REQUIRED)
+├── scripts/              # Implementation scripts
+│   └── <script>.py       # Python for new scripts (ADR-042)
+└── references/           # Optional supporting docs
+```
+
+### SKILL.md Frontmatter
+
+Every SKILL.md MUST include valid YAML frontmatter:
+
+```yaml
+---
+name: skill-name
+version: 1.0.0
+model: claude-sonnet-4-5
+description: >-
+  One-paragraph description of what the skill does.
+  Starts with a verb. Under 200 characters.
+license: MIT
+---
+```
+
+Required fields: `name`, `version`, `description`.
+Optional fields: `model`, `license`.
+
+### Language Policy
+
+| Scenario | Language | Authority |
+|----------|----------|-----------|
+| New scripts | Python (.py) | ADR-042 |
+| Existing PowerShell scripts | PowerShell (.ps1/.psm1) | ADR-042 |
+| Hooks (pre-commit, etc.) | Python (.py) | ADR-042 |
+| Bash scripts | Prohibited | ADR-005, ADR-042 |
+
+**Rationale**: ADR-042 establishes Python-first development. Existing PowerShell scripts are grandfathered.
+
+### Testing Requirements
+
+- Every script MUST have corresponding tests.
+- Python scripts: pytest tests in `tests/` directory.
+- PowerShell scripts: Pester tests co-located with scripts.
+- Target: 80% code coverage minimum.
+- See `testing-approach.md` steering for Pester conventions.
+
+### Scope Control
+
+A skill has one purpose. Signs of scope explosion:
+
+| Signal | Threshold | Action |
+|--------|-----------|--------|
+| File count in PR | > 10 files | Split into separate PRs |
+| Commit count | > 20 commits | Squash or split |
+| Memory file changes | > 0 memory files | Move to separate PR |
+| Multiple unrelated features | > 1 feature | One skill, one PR |
+
+Memory changes (`.serena/memories/`) belong in a **separate PR** from skill implementation.
+
+### Naming Conventions
+
+| Item | Convention | Example |
+|------|-----------|---------|
+| Skill directory | kebab-case | `code-qualities-assessment` |
+| SKILL.md `name` | kebab-case, matches directory | `code-qualities-assessment` |
+| Python scripts | snake_case | `analyze_code.py` |
+| PowerShell scripts | PascalCase with verb-noun | `Get-ApplicableSteering.ps1` |
+| Serena memories | kebab-case with prefix | `skills-github-cli-index` |
+
+## Patterns
+
+### Minimal Skill (Python)
+
+```text
+.claude/skills/my-skill/
+├── SKILL.md
+└── scripts/
+    └── my_skill.py
+```
+
+SKILL.md delegates to the script:
+
+```markdown
+When this skill activates, IMMEDIATELY invoke the script. The script IS the workflow.
+```
+
+### Skill with References
+
+```text
+.claude/skills/prompt-engineer/
+├── SKILL.md
+├── scripts/
+│   └── optimize_prompt.py
+└── references/
+    └── prompt-engineering-patterns.md
+```
+
+Reference files provide domain knowledge. They do not contain executable logic.
+
+## Before PR Checklist
+
+Before submitting a skill PR, verify:
+
+- [ ] SKILL.md has valid frontmatter (name, version, description)
+- [ ] No duplicate skill exists with overlapping purpose
+- [ ] Scripts use Python for new code (ADR-042)
+- [ ] Tests exist and pass
+- [ ] File count in PR is 10 or fewer
+- [ ] Commit count is 20 or fewer
+- [ ] No memory file changes included (separate PR)
+- [ ] Skill directory uses kebab-case naming
+
+## Anti-Patterns
+
+### Scope Explosion
+
+| Anti-Pattern | Problem | Fix |
+|--------------|---------|-----|
+| Skill + hook + memory in one PR | Unbounded scope, hard to review | Separate PRs per concern |
+| Renaming memory conventions mid-PR | Creates cascading changes | Plan naming upfront, use separate PR |
+| Adding "bonus" features | Delays review, introduces risk | One skill, one purpose |
+
+### Missing Validation
+
+| Anti-Pattern | Problem | Fix |
+|--------------|---------|-----|
+| No SKILL.md frontmatter | Tooling cannot discover the skill | Add required YAML fields |
+| No tests | Regressions go undetected | Write tests before or with implementation |
+| Hardcoded paths | Breaks in different environments | Use relative paths from `$PSScriptRoot` or `__file__` |
+
+### Language Violations
+
+| Anti-Pattern | Problem | Fix |
+|--------------|---------|-----|
+| New bash scripts in skills | Violates ADR-005 and ADR-042 | Use Python |
+| New PowerShell for greenfield work | Misses ADR-042 guidance | Use Python for new scripts |
+
+## References
+
+- [ADR-005](../../.agents/architecture/ADR-005-powershell-only-scripting.md): Original PowerShell-only decision (superseded for new development)
+- [ADR-042](../../.agents/architecture/ADR-042-python-migration-strategy.md): Python migration strategy (current)
+- [SKILL-AUTHORING.md](../../docs/SKILL-AUTHORING.md): Skill authoring guide
+- [Steering README](.agents/steering/README.md): Steering system overview
+- Memory: `skills-index`
+
+---
+
+*Created: 2026-02-22*
+*GitHub Issue: #951*

.github/instructions/claude-skills.instructions.md

@@ -0,0 +1,29 @@
+---
+applyTo: ".claude/skills/**"
+---
+
+# Claude Skills Standards
+
+For comprehensive skill development standards, see [.agents/steering/claude-skills.md](../../.agents/steering/claude-skills.md).
+
+## Quick Reference - Skills
+
+**Structure:**
+
+```text
+.claude/skills/<skill-name>/
+├── SKILL.md              # Frontmatter + prompt (REQUIRED)
+├── scripts/              # Implementation scripts
+└── references/           # Optional supporting docs
+```
+
+**Key Principles:**
+
+- One skill, one purpose
+- SKILL.md frontmatter required (name, version, description)
+- Python for new scripts (ADR-042)
+- Tests required for all scripts
+- File count per PR: 10 or fewer
+- Memory changes in separate PR
+
+*This file serves as a Copilot-specific entry point. The authoritative steering content is maintained in `.agents/steering/claude-skills.md`.*