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>

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

@@ -123,6 +123,8 @@ IMPORTANT: If CONTEXT.md exists below, it contains user decisions from /gsd:disc
 **Phase requirement IDs (MUST address):** {phase_req_ids}
 **Requirements:** {requirements}
 **Prior decisions:** {decisions}
+
+**Project skills:** Check .agents/skills/ directory (if exists) — read SKILL.md files, research should account for project skill patterns
 </additional_context>
 
 <output>
@@ -200,6 +202,8 @@ IMPORTANT: If context exists below, it contains USER DECISIONS from /gsd:discuss
 
 **Research:** {research_content}
 **Gap Closure (if --gaps):** {verification_content} {uat_content}
+
+**Project skills:** Check .agents/skills/ directory (if exists) — read SKILL.md files, plans should account for project skill rules
 </planning_context>
 
 <downstream_consumer>
@@ -268,6 +272,8 @@ IMPORTANT: Plans MUST honor user decisions. Flag as issue if plans contradict.
 - **Deferred Ideas** = Out of scope — plans must NOT include
 
 {context_content}
+
+**Project skills:** Check .agents/skills/ directory (if exists) — verify plans account for project skill rules
 </verification_context>
 
 <expected_output>

get-shit-done/workflows/quick.md

@@ -99,6 +99,8 @@ Task(
 **Project State:**
 @.planning/STATE.md
 
+**Project skills:** Check .agents/skills/ directory (if exists) — read SKILL.md files, plans should account for project skill rules
+
 </planning_context>
 
 <constraints>
@@ -249,6 +251,7 @@ Execute quick task ${next_num}.
 
 Plan: @${QUICK_DIR}/${next_num}-PLAN.md
 Project state: @.planning/STATE.md
+Project skills: .agents/skills/ (if exists — list skills, read SKILL.md for each, follow relevant rules during implementation)
 
 <constraints>
 - Execute all tasks in the plan