feat: MCP tool awareness for GSD subagents (#973) (#1137)

GSD executor agents ignore MCP tools (e.g. jCodeMunch) even when CLAUDE.md
explicitly instructs their use. Agents default to Grep/Glob because those
are explicitly referenced in workflow patterns.

Added MCP tool instructions to:
- execute-phase.md: <mcp_tools> section in executor agent prompt telling
  agents to prefer MCP tools over Grep/Glob when available
- execute-plan.md: Step 2 in execute section with MCP tool fallback guidance

Agents now:
1. Check if CLAUDE.md references MCP tools
2. Prefer MCP tools for code navigation when accessible
3. Fall back to Grep/Glob if MCP tools are not available

Fixes #973

get-shit-done/workflows/execute-plan.md

@@ -135,7 +135,8 @@ If previous SUMMARY has unresolved "Issues Encountered" or "Next Phase Readiness
 Deviations are normal — handle via rules below.
 
 1. Read @context files from prompt
-2. Per task:
+2. **MCP tools:** If CLAUDE.md or project instructions reference MCP tools (e.g. jCodeMunch for code navigation), prefer them over Grep/Glob when available. Fall back to Grep/Glob if MCP tools are not accessible.
+3. Per task:
    - **MANDATORY read_first gate:** If the task has a `<read_first>` field, you MUST read every listed file BEFORE making any edits. This is not optional. Do not skip files because you "already know" what's in them — read them. The read_first files establish ground truth for the task.
    - `type="auto"`: if `tdd="true"` → TDD execution. Implement with deviation rules + auth gates. Verify done criteria. Commit (see task_commit). Track hash for Summary.
    - `type="checkpoint:*"`: STOP → checkpoint_protocol → wait for user → continue only after confirmation.