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_global_tuning.py

@@ -12,7 +12,7 @@ admin-only "Tuning" nav link (``#nav-tuning`` — is-active on this page):
 like the Sources link it ships hidden and ``header.js`` reveals it once
 whoami says admin, so an anonymous visitor never sees it.
 
-Story: ``.agent/user_stories/global-tuning.md``
+Story: ``.agents/user_stories/global-tuning.md``
 Run in isolation (DB must be up: ``podman compose up -d db``):
 
     uv run pytest tests/e2e/test_global_tuning.py -v --no-cov