feat(council): add review-plan skill for external plan review before execution

[rube-de]

Context

There is no standalone skill for reviewing implementation plans using external AI consultants before execution begins. The superpowers pipeline (brainstorming → writing-plans → executing-plans) transitions directly from plan writing to execution with no external validation step. Plans may contain stale file paths, flawed assumptions, YAGNI violations, or missed edge cases that fresh-context consultants would catch.

Issue #114 adds opt-in council review to the CDT auto-task workflow specifically. This skill provides the same capability as a standalone, reusable skill that any workflow can invoke — superpowers pipeline, CDT auto-task, manual invocation, or custom workflows.

Task

Create a new council:review-plan skill that:

1. Locates the plan file (argument, docs/plans/ discovery, or conversation context)
2. Verifies plan assumptions against the codebase (file existence, line numbers, API signatures, duplication)
3. Launches gemini-consultant and codex-consultant agents in parallel with a hole-poking prompt
4. Deduplicates and synthesizes findings
5. Presents a structured verdict: Ready / Needs Revision / Needs Discussion

Acceptance Criteria

• VERIFY: Skill file exists at plugins/council/skills/review-plan/SKILL.md
• VERIFY: Skill frontmatter includes name: review-plan and a description that triggers after plan writing
• VERIFY: Skill auto-discovers plan files from docs/plans/ when no argument is provided (most recently modified .md)
• VERIFY: Skill accepts an explicit plan file path as argument
• VERIFY: Codebase verification step checks: files exist, line numbers match, APIs valid, no duplicate implementations, tests don't already exist
• VERIFY: Skill launches gemini-consultant and codex-consultant in parallel using the Agent tool
• VERIFY: Consultant prompt explicitly frames the task as "find problems, not validate"
• VERIFY: Consultant prompt includes the plan content AND codebase verification results
• VERIFY: Consultants rate findings by severity: Critical (blocks execution) / Warning (should address) / Note (nice to know)
• VERIFY: Findings are deduplicated — if both consultants flag the same issue, it's merged with higher confidence
• VERIFY: Consultant disagreements are presented separately
• VERIFY: Output uses structured format: Codebase Verification → Critical → Warnings → Notes → Disagreements → Verdict
• VERIFY: Verdict is one of: Ready to execute / Needs revision / Needs discussion
• VERIFY: "Ready" verdict offers execution handoff (subagent-driven or parallel session)
• VERIFY: "Needs revision" lists specific changes and asks user whether to revise or proceed with known risks
• VERIFY: bun scripts/validate-plugins.mjs passes

Implementation Guide

Files to Create

• plugins/council/skills/review-plan/SKILL.md — The skill definition

Approach

1. Create plugins/council/skills/review-plan/ directory
2. Create SKILL.md with frontmatter:

   name: review-plan
   description: >-
     Review implementation plans using external AI consultants before execution begins.
     Use automatically after writing plans (superpowers:writing-plans) or when the user says
     "review plan", "check my plan", "sanity check the plan", "review the plan before executing".
     Verifies plan assumptions against codebase, launches consultants to find flaws,
     and presents a verdict.
   allowed-tools: [Bash, Read, Grep, Glob, Agent, AskUserQuestion]
   user-invocable: true

3. Skill body follows this workflow:
   • Step 1: Locate plan — Check argument, then ls -t docs/plans/*.md | head -1, then conversation context
   • Step 2: Codebase verification — Glob/Grep/Read to validate all paths, line numbers, and API references in the plan
   • Step 3: Launch consultants — Agent tool with gemini-consultant and codex-consultant in parallel, same prompt to both
   • Step 4: Synthesize — Deduplicate, surface disagreements, merge confidence
   • Step 5: Present — Structured output with verdict
   • Step 6: Decision — Route to execution or revision

Patterns to Follow

• Council skill (plugins/council/skills/council/SKILL.md) for consultant dispatch pattern and rate limit handling
• Council reference (plugins/council/skills/council-reference/SKILL.md) for structured response format
• The consultant agents (plugins/council/agents/gemini-consultant.md, codex-consultant.md) for invocation pattern

Consultant Prompt Template

You are reviewing an implementation plan before execution begins.
Your job is to find problems, not validate.

**The plan:**
[full plan content]

**Codebase verification findings:**
[results from Step 2]

Review critically:
1. Flawed assumptions — What does the plan assume that might be wrong?
2. Missing edge cases — What failure modes or inputs aren't handled?
3. Simpler alternatives — Is there a significantly simpler approach to any task?
4. Dependency risks — What could break between tasks? What ordering issues exist?
5. Security/performance — Any red flags the plan doesn't address?
6. Scope creep — Is the plan doing more than necessary? YAGNI violations?

For each finding, rate severity: Critical / Warning / Note.

Scope

In Scope

• New council:review-plan skill
• Codebase verification against plan contents
• Parallel consultant dispatch (gemini + codex)
• Finding deduplication and synthesis
• Structured verdict output

Out of Scope

• Modifying the superpowers plugin (external, cached)
• Modifying CDT auto-task (feat(cdt): add opt-in council review on plan before dev phase #114 handles that separately)
• Adding more than 2 consultants (gemini + codex is sufficient for plan review; full 5-consultant council is overkill)
• Auto-fixing plan issues (advisory only)

Dependencies

• Related to: feat(cdt): add opt-in council review on plan before dev phase #114 (CDT auto-task could invoke this skill once available)
• Blocks: None
• Blocked by: None

Complexity Hint

• Subsystems touched: council plugin (1 new skill file)
• Structural changes: New skill directory, no modifications to existing files
• Estimated scope: Small — single SKILL.md file following established council patterns

Definition of Done

• Implementation complete
• bun scripts/validate-plugins.mjs passes
• PR approved and merged