chore: remove legacy tools/codex-skills (moved to .agents/skills), split filter drawer tests, clean up stale docs

- tools/codex-skills/ removed — skills now live in .agents/skills/
- docs/reference-agents-*.md removed — replaced by .agents/workflow.md
- FilterDrawerContractTest split into Runtime + Template contract tests
- bin scripts and .gitignore updated for .agents/ paths

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

bin/codex-skills-sync.sh

@@ -44,7 +44,7 @@ fi
 
 script_dir="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)"
 repo_root="$(cd -- "${script_dir}/.." && pwd)"
-source_dir="${repo_root}/tools/codex-skills"
+source_dir="${repo_root}/.agents/skills"
 codex_home_dir="${CODEX_HOME:-${HOME}/.codex}"
 target_dir="${codex_home_dir}/skills"
 managed_skills=(