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.

.gitignore

@@ -16,9 +16,9 @@ htmlcov/
 .env.*
 !.env.example
 
-# --- Planning / agent pipeline state (the .agent/ tree is tracked; only runtime artifacts ignored) ---
-.agent/phase-sessions/
-.agent/pipeline.log
+# --- Planning / agent pipeline state (the .agents/ tree is tracked; only runtime artifacts ignored) ---
+.agents/phase-sessions/
+.agents/pipeline.log
 
 # --- Frontend build artifacts (sources live in frontend/, builds are local-only) ---
 node_modules/