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_background_no_motion.py

@@ -1,6 +1,6 @@
 """Phase 25 E2E (Playwright): the background no longer moves — it only fades.
 
-Story: ``.agent/user_stories/background-no-motion.md`` (supersedes
+Story: ``.agents/user_stories/background-no-motion.md`` (supersedes
 ``background-animation.md``)
 Run in isolation (DB must be up: ``podman compose up -d db``):
 
@@ -12,7 +12,7 @@ It should be smooth, fluxuating, dimming and brightening, but not
 moving. Different bright spots should slowly fade in and out."
 
 The two root causes (phase-22 measurements in
-``.agent/reports/22_background_animation/``):
+``.agents/reports/22_background_animation/``):
   - the "jitter" was the grid's 44px/60s drift (0.73px/s, diagonally
     down-right) — a 1px line translated sub-pixel by sub-pixel
     rasterizes with per-frame stepping, not smooth motion;