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/docs-link-check.sh

@@ -9,9 +9,9 @@ tmp_file="$(mktemp)"
 trap 'rm -f "${tmp_file}"' EXIT
 
 rg -n -o '(?:/docs|docs)/[a-z0-9_/-]+\.md' \
-  README.md docs tools/codex-skills agent-system \
+  README.md docs .agents \
   -g '*.md' \
-  -g '!agent-system/runs/**' \
+  -g '!.agents/runs/**' \
   > "${tmp_file}" || true
 
 missing=0
@@ -33,4 +33,4 @@ if [[ ${missing} -ne 0 ]]; then
   exit 1
 fi
 
-echo "[OK] docs links resolve (agent-system/runs/** excluded by default)"
+echo "[OK] docs links resolve (.agents/runs/** excluded by default)"