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-phase.md

@@ -158,6 +158,13 @@ Execute each wave in sequence. Within a wave: parallel if `PARALLELIZATION=true`
        - .claude/skills/ or .agents/skills/ (Project skills, if either exists — list skills, read SKILL.md for each, follow relevant rules during implementation)
        </files_to_read>
 
+       <mcp_tools>
+       If CLAUDE.md or project instructions reference MCP tools (e.g. jCodeMunch, context7,
+       or other MCP servers), prefer those tools over Grep/Glob for code navigation when available.
+       MCP tools often save significant tokens by providing structured code indexes.
+       Check tool availability first — if MCP tools are not accessible, fall back to Grep/Glob.
+       </mcp_tools>
+
        <success_criteria>
        - [ ] All tasks executed
        - [ ] Each task committed individually