fix(#671): add project CLAUDE.md discovery to subagent spawn points

Subagents now read project-level CLAUDE.md if it exists:
- Workflows: execute-phase, plan-phase, quick
- Agents: gsd-executor, gsd-planner, gsd-phase-researcher, gsd-plan-checker

Agents read ./CLAUDE.md in their fresh context, following project-specific
guidelines, security requirements, and coding conventions.

Fixes: #671

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

agents/gsd-executor.md

@@ -16,6 +16,8 @@ Your job: Execute the plan completely, commit each task, create SUMMARY.md, upda
 <project_context>
 Before executing, discover project context:
 
+**Project instructions:** Read `./CLAUDE.md` if it exists in the working directory. Follow all project-specific guidelines, security requirements, and coding conventions.
+
 **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)

agents/gsd-phase-researcher.md

@@ -21,6 +21,8 @@ Spawned by `/gsd:plan-phase` (integrated) or `/gsd:research-phase` (standalone).
 <project_context>
 Before researching, discover project context:
 
+**Project instructions:** Read `./CLAUDE.md` if it exists in the working directory. Follow all project-specific guidelines, security requirements, and coding conventions.
+
 **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)

agents/gsd-plan-checker.md

@@ -26,6 +26,8 @@ You are NOT the executor or verifier — you verify plans WILL work before execu
 <project_context>
 Before verifying, discover project context:
 
+**Project instructions:** Read `./CLAUDE.md` if it exists in the working directory. Follow all project-specific guidelines, security requirements, and coding conventions.
+
 **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)

agents/gsd-planner.md

@@ -28,6 +28,8 @@ Your job: Produce PLAN.md files that Claude executors can implement without inte
 <project_context>
 Before planning, discover project context:
 
+**Project instructions:** Read `./CLAUDE.md` if it exists in the working directory. Follow all project-specific guidelines, security requirements, and coding conventions.
+
 **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)