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/unit/test_background_animation.py

@@ -32,8 +32,8 @@ spot gradients, reduced motion across all four layers) is pinned in
 tests/unit/test_background_no_motion.py; browser behavior is E2E-covered
 by tests/e2e/test_background_no_motion.py (task 02).
 
-Story: .agent/user_stories/background-no-motion.md (supersedes
-.agent/user_stories/background-animation.md).
+Story: .agents/user_stories/background-no-motion.md (supersedes
+.agents/user_stories/background-animation.md).
 """
 from __future__ import annotations
 

tests/unit/test_background_no_motion.py

@@ -5,7 +5,7 @@ and to the right every second and it slowly blinks brighter and darker.
 It should be smooth, fluxuating, dimming and brightening, but not
 moving. Different bright spots should slowly fade in and out."
 
-The diagnosis (`.agent/reports/25_background_no_motion/`) found both
+The diagnosis (`.agents/reports/25_background_no_motion/`) found both
 root causes in the phase-22 design:
   - "jitters down and to the right" = the grid's 44px/60s drift
     (0.73px/s, diagonally down-right) — a 1px grid line translated
@@ -21,7 +21,7 @@ negative delays → out of phase; LCM 4641s → the composite pattern
 effectively never repeats within a viewing session), so the total light
 fluxuates smoothly and irregularly — no blink, no jitter, no movement.
 
-Story: .agent/user_stories/background-no-motion.md. Browser behavior
+Story: .agents/user_stories/background-no-motion.md. Browser behavior
 (no motion, visible fades, no occlusion, no overflow) is E2E-covered by
 tests/e2e/test_background_no_motion.py (task 02).
 """