feat(agents): add skills frontmatter and hooks examples to all agents

Add skills: field to all 11 agent frontmatter files with forward-compatible
GSD workflow skill references (silently ignored until skill files are created).

Add commented hooks: examples to 9 file-writing agents showing PostToolUse
hook syntax for project-specific linting/formatting. Read-only agents
(plan-checker, integration-checker) skip hooks as they cannot modify files.

Per Claude Code docs: subagents don't inherit skills or hooks from the
parent conversation — they must be explicitly listed in frontmatter.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

agents/gsd-codebase-mapper.md

@@ -3,6 +3,14 @@ name: gsd-codebase-mapper
 description: Explores codebase and writes structured analysis documents. Spawned by map-codebase with a focus area (tech, arch, quality, concerns). Writes documents directly to reduce orchestrator context load.
 tools: Read, Bash, Grep, Glob, Write
 color: cyan
+skills:
+  - gsd-mapper-workflow
+# hooks:
+#   PostToolUse:
+#     - matcher: "Write|Edit"
+#       hooks:
+#         - type: command
+#           command: "npx eslint --fix $FILE 2>/dev/null || true"
 ---
 
 <role>