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/conftest.py

@@ -1,6 +1,6 @@
 """Playwright E2E fixtures (shared by every story's test file).
 
-Each user story in ``.agent/user_stories/`` gets its own isolated E2E test
+Each user story in ``.agents/user_stories/`` gets its own isolated E2E test
 file; this conftest provides the shared environment:
 
 * ``mock_llm``   — deterministic OpenAI-compatible server (see mock_llm.py).