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>

get-shit-done/templates/state.md

@@ -60,6 +60,12 @@ Recent decisions affecting current work:
 
 None yet.
 
+### Pending Todos
+
+[From .planning/todos/pending/ — ideas captured during sessions]
+
+None yet.
+
 ### Blockers/Concerns
 
 [Issues that affect future work]
@@ -152,6 +158,11 @@ Updated after each plan completion.
 - Effort estimate if known
 - Helps phase planning identify what to address
 
+**Pending Todos:** Ideas captured via /gsd:add-todo
+- Count of pending todos
+- Reference to .planning/todos/pending/
+- Brief list if few, count if many (e.g., "5 pending todos — see /gsd:check-todos")
+
 **Blockers/Concerns:** From "Next Phase Readiness" sections
 - Issues that affect future work
 - Prefix with originating phase