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.md

@@ -1,18 +1,18 @@
 # AGENTS.md — Operating Rules for All Agents on This Repo
 
-1. **Always read `.agent/PLAN.md` first.** It contains the architecture, the
+1. **Always read `.agents/PLAN.md` first.** It contains the architecture, the
    LOCKED DECISIONS, the UI/UX standards, the data model, and the roadmap.
    Do not design anything that contradicts it.
-2. **Follow the phased execution protocol in `.agent/phases/`.** Work through
-   `.agent/phases/todo/` in numeric order. When a phase is complete, move its
-   file to `.agent/phases/complete/`.
+2. **Follow the phased execution protocol in `.agents/phases/`.** Work through
+   `.agents/phases/todo/` in numeric order. When a phase is complete, move its
+   file to `.agents/phases/complete/`.
 3. **Strictly adhere to the LOCKED DECISIONS** (PLAN §2). If you believe a
    LOCKED decision is wrong, stop and flag it — do not silently deviate.
-4. **"One Story, One Phase":** each user story in `.agent/user_stories/`
+4. **"One Story, One Phase":** each user story in `.agents/user_stories/`
    corresponds to a distinct execution phase that includes its own dedicated
    Playwright E2E test suite (one test file per story, run in isolation).
 5. **"UI Structure Check":** before finalizing any UI component, verify that
-   it follows the layout principles in `.agent/PLAN.md` §7 (proper container
+   it follows the layout principles in `.agents/PLAN.md` §7 (proper container
    usage, centered 46rem chat column, full-width tables on Sources — no
    skinny wasted-space lists) and meets WCAG 2.1 AA basics (semantic
    landmarks, labels, contrast ≥4.5:1, focus-visible, aria-live for the
@@ -27,9 +27,9 @@
 8. **Git protocol:** Git is mandatory. One **atomic, professional commit per
    completed phase** (Conventional Commits, e.g. `feat(rag): stream RAG
    answers over SSE with source citations`). Always append `--no-gpg-sign`
-   (the repo also sets `commit.gpgsign=false`). `.agent/` is **tracked and
-   committed** with the phase — only runtime artifacts (`.agent/phase-sessions/`,
-   `.agent/pipeline.log`) are gitignored.
+   (the repo also sets `commit.gpgsign=false`). `.agents/` is **tracked and
+   committed** with the phase — only runtime artifacts (`.agents/phase-sessions/`,
+   `.agents/pipeline.log`) are gitignored.
 9. **Test gates are non-negotiable:** a phase is complete only when unit +
    integration tests pass, `app/` coverage is **>90%**
    (`uv run pytest --cov=app --cov-report=term-missing`), and the phase's