feat: enhance artifact instructions with inline guidance and XML output (#422)

* feat: enhance artifact instructions with inline guidance and XML output

Schema changes:
- Add `instruction` field to artifacts for inline creation guidance
- Include detailed instructions for proposal, specs, design, and tasks

Instruction loader enhancements:
- Return `instruction` field from schema
- Include `changeDir` for full path resolution
- Enrich dependency info with `path` and `description` fields

CLI output improvements:
- New XML-style format for `openspec instructions` (better for AI parsing)
- Structured tags: <artifact>, <task>, <context>, <output>, <template>
- Dependencies now show full paths for easy file reading
- JSON output includes all new fields

* docs: add schema customization and workflow gap documentation

- schema-customization.md: Guide for customizing artifact schemas
- schema-workflow-gaps.md: Analysis of current workflow limitations

* test: fix list test for new default sort order

Update test to explicitly use sort='name' since the default
changed from alphabetical to most-recently-modified.

Author: TabishB

docs/schema-customization.md

@@ -0,0 +1,211 @@
+# Schema Customization
+
+This document describes how users can customize OpenSpec schemas and templates, the current manual process, and the gap that needs to be addressed.
+
+---
+
+## Overview
+
+OpenSpec uses a 2-level schema resolution system following the XDG Base Directory Specification:
+
+1. **User override**: `${XDG_DATA_HOME}/openspec/schemas/<name>/`
+2. **Package built-in**: `<npm-package>/schemas/<name>/`
+
+When a schema is requested (e.g., `spec-driven`), the resolver checks the user directory first. If found, that entire schema directory is used. Otherwise, it falls back to the package's built-in schema.
+
+---
+
+## Current Manual Process
+
+To override the default `spec-driven` schema, a user must:
+
+### 1. Determine the correct directory path
+
+| Platform | Path |
+|----------|------|
+| macOS/Linux | `~/.local/share/openspec/schemas/` |
+| Windows | `%LOCALAPPDATA%\openspec\schemas\` |
+| All (if set) | `$XDG_DATA_HOME/openspec/schemas/` |
+
+### 2. Create the directory structure
+
+```bash
+# macOS/Linux example
+mkdir -p ~/.local/share/openspec/schemas/spec-driven/templates
+```
+
+### 3. Find and copy the default schema files
+
+The user must locate the installed npm package to copy the defaults:
+
+```bash
+# Find the package location (varies by install method)
+npm list -g openspec --parseable
+# or
+which openspec && readlink -f $(which openspec)
+
+# Copy files from the package's schemas/ directory
+cp <package-path>/schemas/spec-driven/schema.yaml ~/.local/share/openspec/schemas/spec-driven/
+cp <package-path>/schemas/spec-driven/templates/*.md ~/.local/share/openspec/schemas/spec-driven/templates/
+```
+
+### 4. Modify the copied files
+
+Edit `schema.yaml` to change the workflow structure:
+
+```yaml
+name: spec-driven
+version: 1
+description: My custom workflow
+artifacts:
+  - id: proposal
+    generates: proposal.md
+    description: Initial proposal
+    template: proposal.md
+    requires: []
+  # Add, remove, or modify artifacts...
+```
+
+Edit templates in `templates/` to customize the content guidance.
+
+### 5. Verify the override is active
+
+Currently there's no command to verify which schema is being used. Users must trust that the file exists in the right location.
+
+---
+
+## Gap Analysis
+
+The current process has several friction points:
+
+| Issue | Impact |
+|-------|--------|
+| **Path discovery** | Users must know XDG conventions and platform-specific paths |
+| **Package location** | Finding the npm package path varies by install method (global, local, pnpm, yarn, volta, etc.) |
+| **No scaffolding** | Users must manually create directories and copy files |
+| **No verification** | No way to confirm which schema is actually being resolved |
+| **No diffing** | When upgrading openspec, users can't see what changed in built-in templates |
+| **Full copy required** | Must copy entire schema even to change one template |
+
+### User Stories Not Currently Supported
+
+1. *"I want to add a `research` artifact before `proposal`"* — requires manual copy and edit
+2. *"I want to customize just the proposal template"* — must copy entire schema
+3. *"I want to see what the default schema looks like"* — must find package path
+4. *"I want to revert to defaults"* — must delete files and hope paths are correct
+5. *"I upgraded openspec, did the templates change?"* — no way to diff
+
+---
+
+## Proposed Solution: Schema Configurator
+
+A CLI command (or set of commands) that handles path resolution and file operations for users.
+
+### Option A: Single `openspec schema` command
+
+```bash
+# List available schemas (built-in and user overrides)
+openspec schema list
+
+# Show where a schema resolves from
+openspec schema which spec-driven
+# Output: /Users/me/.local/share/openspec/schemas/spec-driven/ (user override)
+# Output: /usr/local/lib/node_modules/openspec/schemas/spec-driven/ (built-in)
+
+# Copy a built-in schema to user directory for customization
+openspec schema copy spec-driven
+# Creates ~/.local/share/openspec/schemas/spec-driven/ with all files
+
+# Show diff between user override and built-in
+openspec schema diff spec-driven
+
+# Remove user override (revert to built-in)
+openspec schema reset spec-driven
+
+# Validate a schema
+openspec schema validate spec-driven
+```
+
+### Option B: Dedicated `openspec customize` command
+
+```bash
+# Interactive schema customization
+openspec customize
+# Prompts: Which schema? What do you want to change? etc.
+
+# Copy and open for editing
+openspec customize spec-driven
+# Copies to user dir, prints path, optionally opens in $EDITOR
+```
+
+### Option C: Init-time schema selection
+
+```bash
+# During project init, offer schema customization
+openspec init
+# ? Select a workflow schema:
+#   > spec-driven (default)
+#     tdd
+#     minimal
+#     custom (copy and edit)
+```
+
+### Recommended Approach
+
+**Option A** provides the most flexibility and follows Unix conventions (subcommands for discrete operations). Key commands in priority order:
+
+1. `openspec schema list` — see what's available
+2. `openspec schema which <name>` — debug resolution
+3. `openspec schema copy <name>` — scaffold customization
+4. `openspec schema diff <name>` — compare with built-in
+5. `openspec schema reset <name>` — revert to defaults
+
+---
+
+## Implementation Considerations
+
+### Path Resolution
+
+The resolver already exists in `src/core/artifact-graph/resolver.ts`:
+
+```typescript
+export function getPackageSchemasDir(): string { ... }
+export function getUserSchemasDir(): string { ... }
+export function getSchemaDir(name: string): string | null { ... }
+export function listSchemas(): string[] { ... }
+```
+
+New commands would leverage these existing functions.
+
+### File Operations
+
+- Copy should preserve file permissions
+- Copy should not overwrite existing user files without `--force`
+- Reset should prompt for confirmation
+
+### Template-Only Overrides
+
+A future enhancement could support overriding individual templates without copying the entire schema. This would require changes to the resolution logic:
+
+```
+Current: schema dir (user) OR schema dir (built-in)
+Future:  schema.yaml from user OR built-in
+         + each template from user OR built-in (independent fallback)
+```
+
+This adds complexity but enables the "I just want to change one template" use case.
+
+---
+
+## Related Documents
+
+- [Schema Workflow Gaps](./schema-workflow-gaps.md) — End-to-end workflow analysis and phased implementation plan
+
+## Related Files
+
+| File | Purpose |
+|------|---------|
+| `src/core/artifact-graph/resolver.ts` | Schema resolution logic |
+| `src/core/artifact-graph/instruction-loader.ts` | Template loading |
+| `src/core/global-config.ts` | XDG path helpers |
+| `schemas/spec-driven/` | Default schema and templates |