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.

.agents/reports/02_story_import_documents.a2.err

@@ -0,0 +1,5 @@
+No API key found for the selected model.
+
+Use /login to log into a provider via OAuth or API key. See:
+  /var/home/ducoterra/.nvm/versions/node/v24.18.0/lib/node_modules/@earendil-works/pi-coding-agent/docs/providers.md
+  /var/home/ducoterra/.nvm/versions/node/v24.18.0/lib/node_modules/@earendil-works/pi-coding-agent/docs/models.md