fix: enforce plan file naming convention in gsd-planner agent

gsd-tools detects plan files by matching the glob `*-PLAN.md`
(e.g. 01-01-PLAN.md). When the planner generates files using a
different convention — wave-based names, wrong prefix order, or
lowercase — the tool returns plan_count: 0 and execution cannot
proceed.

Root cause: the write_phase_prompt step only said
"Write to .../XX-name/{phase}-{NN}-PLAN.md" — ambiguous enough
for the agent to produce PLAN-01-auth.md, 01-PLAN-01.md, etc.

Observed across real usage (306 sessions analyzed):
- Phases 2, 3, 4: plan files used wave-based names instead of the
  numeric format gsd-tools expects; required manual detection and
  adaptation before execution could proceed each time
- gsd-tools roadmap get-phase failed on Phase 3 due to format
  mismatch; Claude fell back to parsing ROADMAP.md manually
- Naming mismatch caused friction in at least 4 separate sessions,
  each requiring a manual workaround

Fix: add a CRITICAL naming block in write_phase_prompt with the
exact required pattern, component definitions, correct/incorrect
examples, and explicit ❌ markers for variants that break detection.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

agents/gsd-planner.md

@@ -1193,7 +1193,26 @@ Use template structure for each PLAN.md.
 
 **ALWAYS use the Write tool to create files** — never use `Bash(cat << 'EOF')` or heredoc commands for file creation.
 
-Write to `.planning/phases/XX-name/{phase}-{NN}-PLAN.md`
+**CRITICAL — File naming convention (enforced):**
+
+The filename MUST follow the exact pattern: `{padded_phase}-{NN}-PLAN.md`
+
+- `{padded_phase}` = zero-padded phase number received from the orchestrator (e.g. `01`, `02`, `03`, `02.1`)
+- `{NN}` = zero-padded sequential plan number within the phase (e.g. `01`, `02`, `03`)
+- The suffix is always `-PLAN.md` — NEVER `PLAN-NN.md`, `NN-PLAN.md`, or any other variation
+
+**Correct examples:**
+- Phase 1, Plan 1 → `01-01-PLAN.md`
+- Phase 3, Plan 2 → `03-02-PLAN.md`
+- Phase 2.1, Plan 1 → `02.1-01-PLAN.md`
+
+**Incorrect (will break gsd-tools detection):**
+- ❌ `PLAN-01-auth.md`
+- ❌ `01-PLAN-01.md`
+- ❌ `plan-01.md`
+- ❌ `01-01-plan.md` (lowercase)
+
+Full write path: `.planning/phases/{padded_phase}-{slug}/{padded_phase}-{NN}-PLAN.md`
 
 Include all frontmatter fields.
 </step>