feat(#776): Gemini hook events (BeforeAgent/AfterAgent/BeforeModel) + hooksConfig.enabled check (#829)

* feat(#776): Gemini hook events (BeforeAgent/AfterAgent/BeforeModel) + hooksConfig.enabled check

Register three new Gemini-CLI hook events on install:
  - BeforeAgent: fires before agent planning; wired to gsd-context-monitor
  - AfterAgent: fires after final response generation; wired to gsd-context-monitor
  - BeforeModel: fires before each LLM call (per-turn); wired to gsd-context-monitor

All three reuse gsd-context-monitor.js (no new hook files). Uninstall cleanup
loop extended to remove the new events. Non-array guard added for robustness
against malformed settings.

Also detect hooksConfig.enabled:false in Gemini settings and emit a clear
warning — without this check, all registered hooks silently do nothing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore: update changeset pr: 829

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* docs(#776): document Gemini hook events

Add hook coverage table to the Gemini CLI section of install-on-your-runtime.md,
covering the three new events (BeforeAgent/AfterAgent/BeforeModel wired to
gsd-context-monitor) plus a callout for the hooksConfig.enabled:false silent
failure mode detected by the installer.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

scripts/lint-test-file-count.allowlist.json

@@ -121,6 +121,7 @@
     "install": {
       "files": [
         "bug-410-install-defaults-test-mode-guard.test.cjs",
+        "enh-776-install-gemini-hook-events.test.cjs",
         "install-minimal-hooks.test.cjs",
         "install-path-detection.test.cjs",
         "install-regressions.test.cjs",