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>

README.md

@@ -243,6 +243,7 @@ GSD handles it for you:
 | `PLAN.md` | Atomic task with XML structure, verification steps |
 | `SUMMARY.md` | What happened, what changed, committed to history |
 | `ISSUES.md` | Deferred enhancements tracked across sessions |
+| `todos/` | Captured ideas and tasks for later work |
 
 Size limits based on where Claude's quality degrades. Stay under, get consistent excellence.
 
@@ -332,6 +333,8 @@ You're never locked in. The system adapts.
 | `/gsd:resume-work` | Restore from last session |
 | `/gsd:resume-task [id]` | Resume interrupted subagent execution |
 | `/gsd:consider-issues` | Review deferred issues, close resolved, identify urgent |
+| `/gsd:add-todo [desc]` | Capture idea or task from conversation for later |
+| `/gsd:check-todos [area]` | List pending todos, select one to work on |
 | `/gsd:help` | Show all commands and usage guide |
 
 <sup>¹ Contributed by reddit user OracleGreyBeard</sup>