refactor(agents): migrate .agent/ planning tree to .agents/

Standardize on the .agents/ directory (shared with project skills):
phases/, user_stories/, reports/, screenshots/, validate.sh, and
phase-sessions/ + pipeline.log all move to .agents/ (git mv preserves
history; runtime artifacts move alongside).

Updates every reference in AGENTS.md, README.md, .gitignore, app
docstrings, and test story headers. Historical KB content in data/
and the runtime pipeline.log transcript are left untouched.

tests/e2e/test_sources_midstream_bug.py

@@ -1,6 +1,6 @@
 """Phase 20 E2E (Playwright): navigating away mid-turn keeps the answer.
 
-Story: ``.agent/user_stories/sources-midstream.md``
+Story: ``.agents/user_stories/sources-midstream.md``
 Bug report (TODO.md L3): *"Clicking "sources" while chat is generating
 clears chat and result will never show up."*
 Run in isolation (DB must be up: ``podman compose up -d db``):