fix(#1437): add phase.list-plans to gsd-tools (#1485)

* fix(#1437): add phase.list-plans to gsd-tools

Register phase.list-plans in PHASE_COMMAND_ALIASES, implement
cmdPhaseListPlans in src/phase.cts (uses findPhaseInternal + scanPhasePlans
to return plan_count/has_plans/plans/phase_dir), and wire the handler in
phase-command-router. Previously every call produced "Unknown phase
subcommand".

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

* fix(#1437): register new test file in lint-test-file-count allowlist

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

* fix(#1437): rename test to fix-NNN convention; update file-count allowlist

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

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.changeset/sturdy-wasps-swim.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 1437
+---
+add phase.list-plans to gsd-tools — the command was referenced in agents/gsd-plan-checker.md but was missing from the router, causing 'Unknown phase subcommand' on every invocation