fix(#2418,#2399,#2419,#2421): four workflow and installer bug fixes (#2462)

- #2418: convertClaudeToAntigravityContent now replaces bare ~/.claude and
  $HOME/.claude (no trailing slash) for both global and local installs,
  eliminating the "unreplaced .claude path reference" warnings in
  gsd-debugger.md and update.md during Antigravity installs.

- #2399: plan-phase workflow gains step 13c that commits PLAN.md files
  and STATE.md via gsd-sdk query commit when commit_docs is true.
  Previously commit_docs:true was read but never acted on in plan-phase.

- #2419: new-project.md and new-milestone.md now parse agents_installed
  and missing_agents from the init JSON and warn users clearly when GSD
  agents are not installed, rather than silently failing with "agent type
  not found" when trying to spawn gsd-project-researcher subagents.

- #2421: gsd-planner.md gains a "Grep gate hygiene" rule immediately after
  the Nyquist Rule explaining the self-invalidating grep gate anti-pattern
  and providing comment-stripping alternatives (grep -v, ast-grep).

Tests: 4 new test files (30 tests) all passing.

Closes #2418
Closes #2399
Closes #2419
Closes #2421

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

agents/gsd-planner.md

@@ -215,6 +215,8 @@ Every task has four required fields:
 
 **Nyquist Rule:** Every `<verify>` must include an `<automated>` command. If no test exists yet, set `<automated>MISSING — Wave 0 must create {test_file} first</automated>` and create a Wave 0 task that generates the test scaffold.
 
+**Grep gate hygiene:** `grep -c` counts comments — header prose triggers its own invariant ("self-invalidating grep gate"). Use `grep -v '^#' | grep -c token`. Bare `== 0` gates on unfiltered files are forbidden.
+
 **<done>:** Acceptance criteria - measurable state of completion.
 - Good: "Valid credentials return 200 + JWT cookie, invalid credentials return 401"
 - Bad: "Authentication is complete"