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.

README.md

@@ -21,8 +21,8 @@ If it doesn't have notes for your question, it admits it:
 - **Stack:** FastAPI · Pydantic v2 · SQLAlchemy 2 · Alembic · pgvector ·
   vanilla HTML/CSS/JS (no CDN) · Playwright E2E
 - **Planning:** architecture, LOCKED decisions and the phase roadmap live
-  in [`.agent/PLAN.md`](.agent/PLAN.md); per-story specs in
-  [`.agent/user_stories/`](.agent/user_stories/).
+  in [`.agents/PLAN.md`](.agents/PLAN.md); per-story specs in
+  [`.agents/user_stories/`](.agents/user_stories/).
 
 ---
 
@@ -706,7 +706,7 @@ uv run playwright install chromium
 podman compose up -d db
 uv run pytest tests/e2e/test_import_documents.py -v --no-cov
 uv run pytest tests/e2e/test_chat_rag.py -v --no-cov
-# ...one file per story in .agent/user_stories/ (see .agent/phases/todo/)
+# ...one file per story in .agents/user_stories/ (see .agents/phases/todo/)
 ```
 
 **Deterministic E2E:** by default the E2E app talks to a local **mock