fix(3605): scrub retired slash commands from agents/*.md

Six surviving references to /gsd-research-phase (deleted in #3042) and
/gsd-insert-phase (consolidated into /gsd:phase insert in v1.40.0)
remained in five agent contracts because every prior scrub pass (#3029,
#3044, #3131) limited its SEARCH_DIRS to workflows/, references/,
templates/, contexts/, commands/, and hooks/ — agents/ was outside scope.
agents/gsd-executor.md:195 is user-facing: the executor surfaces it
during a package-install failure recovery checkpoint, so a real user
hits "Unknown command" while trying to recover from a stalled phase.

Replacements:
- /gsd-research-phase -> /gsd:plan-phase --research-phase <N>
  (agents/gsd-executor.md:195, agents/gsd-phase-researcher.md:17,
   agents/gsd-planner.md:186, agents/gsd-planner.md:991,
   agents/gsd-research-synthesizer.md:115)
- /gsd-insert-phase -> /gsd:phase insert
  (agents/gsd-roadmapper.md:205)

Adds tests/bug-3605-stale-research-insert-phase-agent-refs.test.cjs as
the regression guard. It scans agents/*.md for any retired command name
(/gsd-research-phase, /gsd-insert-phase, /gsd-add-phase,
/gsd-remove-phase, /gsd-analyze-dependencies) with proper word-boundary
matching so a future consolidation that misses agents/ fails CI.

The guard mirrors tests/bug-2950-stale-command-refs.test.cjs which
covers the same anti-pattern for workflows/.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

agents/gsd-roadmapper.md

@@ -202,7 +202,7 @@ Track coverage as you go.
 **Integer phases (1, 2, 3):** Planned milestone work.
 
 **Decimal phases (2.1, 2.2):** Urgent insertions after planning.
-- Created via `/gsd-insert-phase`
+- Created via `/gsd:phase insert`
 - Execute between integers: 1 → 1.1 → 1.2 → 2
 
 **Starting number:**