enhancement(workflow): add decision IDs for discuss-to-plan traceability (#1243)

Decisions in CONTEXT.md are now numbered (D-01, D-02, etc.) so
downstream agents can reference them and the plan-checker can verify
100% coverage.

Changes:
- templates/context.md: Decisions use **D-XX:** prefix format
- workflows/discuss-phase.md: write_context step numbers decisions
- agents/gsd-planner.md: Self-check verifies decision ID references
  in task actions; tasks reference D-XX IDs for traceability
- agents/gsd-plan-checker.md: Dimension 7 (Context Compliance)
  extracts D-XX IDs and verifies every decision has a task

get-shit-done/workflows/discuss-phase.md

@@ -650,11 +650,11 @@ mkdir -p ".planning/phases/${padded_phase}-${phase_slug}"
 ## Implementation Decisions
 
 ### [Category 1 that was discussed]
-- [Decision or preference captured]
-- [Another decision if applicable]
+- **D-01:** [Decision or preference captured]
+- **D-02:** [Another decision if applicable]
 
 ### [Category 2 that was discussed]
-- [Decision or preference captured]
+- **D-03:** [Decision or preference captured]
 
 ### Claude's Discretion
 [Areas where user said "you decide" — note that Claude has flexibility here]