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>

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

@@ -121,6 +121,7 @@ Execute each wave in sequence. Within a wave: parallel if `PARALLELIZATION=true`
        - Plan: {phase_dir}/{plan_file}
        - State: .planning/STATE.md
        - Config: .planning/config.json (if exists)
+       - Project skills: .agents/skills/ (if exists — list skills, read SKILL.md for each, follow relevant rules during implementation)
        </files_to_read>
 
        <success_criteria>