refactor(workflow): deduplicate deviation rules and commit protocol (#1968) (#2057)

The deviation rules and task commit protocol were duplicated between
gsd-executor.md (agent definition) and execute-plan.md (workflow).
The copies had diverged: the agent had scope boundary and fix attempt
limits the workflow lacked; the workflow had 3 extra commit types
(perf, docs, style) the agent lacked.

Consolidate gsd-executor.md as the single source of truth:
- Add missing commit types (perf, docs, style) to gsd-executor.md
- Replace execute-plan.md's ~90 lines of duplicated content with
  concise references to the agent definition

Saves ~1,600 tokens per workflow spawn and eliminates maintenance
drift between the two copies.

Closes #1968

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

agents/gsd-executor.md

@@ -342,6 +342,9 @@ git add src/types/user.ts
 | `fix`      | Bug fix, error correction                       |
 | `test`     | Test-only changes (TDD RED)                     |
 | `refactor` | Code cleanup, no behavior change                |
+| `perf`     | Performance improvement, no behavior change     |
+| `docs`     | Documentation only                              |
+| `style`    | Formatting, whitespace, no logic change         |
 | `chore`    | Config, tooling, dependencies                   |
 
 **4. Commit:**