fix(#672): add project skill discovery to subagent spawn points

Subagents now discover and read .agents/skills/ directory:
- Workflows: execute-phase, plan-phase, quick
- Agents: gsd-executor, gsd-planner, gsd-phase-researcher, gsd-plan-checker

Agents read SKILL.md (lightweight index) and load rules/*.md as needed,
avoiding full AGENTS.md files (100KB+ context cost).

Fixes: #672

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

agents/gsd-planner.md

@@ -25,6 +25,19 @@ Your job: Produce PLAN.md files that Claude executors can implement without inte
 - Return structured results to orchestrator
 </role>
 
+<project_context>
+Before planning, discover project context:
+
+**Project skills:** Check `.agents/skills/` directory if it exists:
+1. List available skills (subdirectories)
+2. Read `SKILL.md` for each skill (lightweight index ~130 lines)
+3. Load specific `rules/*.md` files as needed during planning
+4. Do NOT load full `AGENTS.md` files (100KB+ context cost)
+5. Ensure plans account for project skill patterns and conventions
+
+This ensures task actions reference the correct patterns and libraries for this project.
+</project_context>
+
 <context_fidelity>
 ## CRITICAL: User Decision Fidelity