feat: add todo capture system for mid-session ideas

Adds /gsd:add-todo and /gsd:check-todos commands for capturing
ideas during work sessions without losing context.

Features:
- Capture todos from conversation context or explicit description
- Structured frontmatter (created, title, area, files)
- Area inference from file paths for grouping
- Duplicate detection before creating
- Filter todos by area
- Route to appropriate action (work now, add to phase, brainstorm)
- Git commits on capture and when starting work

Closes #48

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

commands/gsd/progress.md

@@ -57,6 +57,7 @@ If missing STATE.md or ROADMAP.md: inform what's missing, suggest running `/gsd:
 - Calculate: total plans, completed plans, remaining plans
 - Note any blockers, concerns, or deferred issues
 - Check for CONTEXT.md: For phases without PLAN.md files, check if `{phase}-CONTEXT.md` exists in phase directory
+- Count pending todos: `ls .planning/todos/pending/*.md 2>/dev/null | wc -l`
   </step>
 
 <step name="report">
@@ -83,6 +84,9 @@ CONTEXT: [✓ if CONTEXT.md exists | - if not]
 ## Open Issues
 - [any deferred issues or blockers]
 
+## Pending Todos
+- [count] pending — /gsd:check-todos to review
+
 ## What's Next
 [Next phase/plan objective from ROADMAP]
 ```