[FEATURE] Add Executive Summary Section to Plan Mode Output

Problem

Plan mode creates detailed plans (400-1900+ lines) that mix user-facing decisions with technical implementation details. When managing multiple Claude Code sessions, reviewing these plans becomes a bottleneck:

• Current: 5-10 minutes to review a plan, scanning for critical decisions buried in technical details
• Impact: Users trust Claude to execute well but need quick review of approach, risks, and low-confidence items

Proposed Solution

Add a mandatory "Summary for Review" section at the top of all plan files:

# [Plan Title]

## Summary for Review

### Approach
[One sentence describing the strategy]

### Critical Decisions
- [Architectural choices that impact scope/behavior]

### Low Confidence Items ⚠️
- [Things Claude is uncertain about and needs validation]

### User Input Needed
- [Specific questions requiring user answers]

### Risks & Tradeoffs
- [What could go wrong, what you're giving up]

---

## Detailed Implementation Plan
[Full technical context for execution]

Benefits

1. Faster review: 30 seconds to scan summary vs 10 minutes for full plan
2. Better decisions: Critical items highlighted, not buried in implementation details
3. Maintained quality: Claude still has full technical context below for execution
4. Multiple sessions: Users can quickly triage which plans need deep review

Implementation

Location: System instructions → Plan Mode → Phase 4: Final Plan

Change:

### Phase 4: Final Plan
Goal: Write your final plan to the plan file (the only file you can edit).
- Include only your recommended approach, not all alternatives
-- Ensure that the plan file is concise enough to scan quickly, but detailed enough to execute effectively
++ **CRITICAL: Start with a "Summary for Review" section**
++   - Template: Approach (1 sentence), Critical Decisions, Low Confidence Items, User Input Needed, Risks & Tradeoffs
++   - Keep summary to 10-20 lines max
++   - Separate from detailed plan with \`---\`
++   - Detailed plan below contains all technical context needed for execution
- Include the paths of critical files to be modified

Real-World Example

I've been using Claude Code extensively for PKM system development and recently implemented this format in my CLAUDE.md instructions. The improvement is immediate:

• Before: 1900-line Bitcoin Temperature plan took 10+ minutes to review
• After: 20-line summary gives full context in 30 seconds, detailed plan below for execution

Why This Matters

Plan mode is powerful but can become a review bottleneck. This enhancement makes plans scannable while preserving all technical detail Claude needs. It's especially valuable for users managing multiple concurrent Claude Code sessions.

Related Issues

This is distinct from existing plan mode issues which focus on safety (#5406, #7474) and configurability (#2798, #12707). This addresses plan review efficiency, not plan mode behavior.