docs: add Cognitive Mode Switching workflow + gstack integration (v3.34.9)

- guide/workflows/gstack-workflow.md (new): Cognitive Mode Switching
  pattern — 6 gears table, pre-implementation strategic gate concept,
  /browse non-MCP native Chromium daemon architecture (~100ms/cmd),
  full ship cycle demo. Reference impl: gstack by Garry Tan (YC CEO).
- examples/commands/plan-ceo-review.md (new): strategic product gate
  template with 3 modes (SCOPE EXPANSION / HOLD SCOPE / REDUCTION)
- examples/commands/plan-eng-review.md (new): engineering architecture
  gate template with Mermaid diagrams, failure modes, test matrix
- guide/workflows/README.md: add entry + 2 Quick Selection Guide rows
- guide/ecosystem/third-party-tools.md: gstack in Plugin Ecosystem
- machine-readable/reference.yaml: v3.34.9, 11 new gstack entries

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

machine-readable/reference.yaml

@@ -330,6 +330,10 @@ deep_dive:
   third_party_tools_rtk: "guide/ecosystem/third-party-tools.md:7"
   third_party_tools_session_mgmt: "guide/ecosystem/third-party-tools.md:126"
   third_party_tools_config_mgmt: "guide/ecosystem/third-party-tools.md:214"
+  third_party_tools_hook_utilities: "guide/ecosystem/third-party-tools.md:331"
+  third_party_gitdiff_watcher: "guide/ecosystem/third-party-tools.md:335"
+  third_party_gitdiff_watcher_upstream: "https://github.com/fcamblor/gitdiff-watcher"
+  third_party_gitdiff_watcher_purpose: "Stop hook utility — enforces build/test quality gates before handoff, fires only when matching files changed"
   third_party_tools_alternative_uis: "guide/ecosystem/third-party-tools.md:259"
   third_party_tools_known_gaps: "guide/ecosystem/third-party-tools.md:375"
   third_party_tools_recommendations: "guide/ecosystem/third-party-tools.md:391"