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/phases/complete/21_thinking_no_scroll/00_phase.md

@@ -0,0 +1,104 @@
+# Phase 21 — Thinking Window: No Scroll Back, Just Live
+
+**Source:** `TODO.md` L4 — *"Disable scroll in the thinking window.
+Users don't need to scroll back through thinking, just see it live."*
+**Story:** `.agents/user_stories/thinking-no-scroll.md` (created by task 02)
+**Context:** `frontend/assets/styles.css` —
+`details.thinking .thinking-text` (today: `max-height: 320px;
+overflow-y: auto;`, i.e. a user-scrollable 320px window, ~line 444);
+`frontend/assets/app.js` — the phase-17 thinking block (`ensureThinkingBlock`,
+the streaming `thinking` branch that appends chunks and already pins the
+stream to the bottom: `textEl.scrollTop = textEl.scrollHeight`, ~line 886),
+and the phase-14 restore path (stored `thinking` re-renders a
+**collapsed** block).
+
+## Objective
+The live Thinking block is a scratchpad, not a transcript. The user must
+not be able to scroll back through it — the 320px window always shows the
+**live tail** of the reasoning stream (the existing per-chunk
+bottom-pinning stays). Wheel, drag, and keyboard scrolling on
+`.thinking-text` stop working; the stream itself keeps pinning to the
+bottom as chunks arrive.
+
+## Owner-confirmed (2026-08-24, roadmap A2)
+1. **Keep the 320px clip** — "just see it live" means the window stays a
+   fixed 320px viewport showing the newest lines; no auto-height growth,
+   no "↓ more" affordance.
+2. **The answer bubble is untouched** — final answers keep their existing
+   scroll behavior (phase 11 long answers).
+3. **Restored (collapsed) Thinking blocks are untouched** — the phase-14
+   restore renders them collapsed, where overflow is moot.
+
+## Design
+- **CSS (the whole functional change):**
+  `details.thinking .thinking-text` — `overflow-y: auto` →
+  `overflow-y: hidden`; keep `max-height: 320px`.
+  `overflow: hidden` still permits **programmatic** scrolling
+  (`scrollTop`), so the phase-17 pin
+  (`textEl.scrollTop = textEl.scrollHeight` on every thinking chunk)
+  keeps the window glued to the live tail — no JS change needed.
+  Add a CSS comment: *no user scroll back (owner choice 2026-08-24):
+  the window is a live tail only — the JS bottom-pin is the sole
+  scroller*.
+- **No JS change** — the pin already exists; nothing else touches
+  `.thinking-text` scroll.
+- **Non-goals:** no change to the answer bubble, the collapsed restore
+  state, the summary/chevron, or the auto-collapse on first delta
+  (phase 17).
+
+## Dependencies
+- `17_thinking_display` (complete) — the block, the pin, the restore.
+- `18_follow_bottom_scroll` (complete) — no overlap (chat-page scroll
+  gate only; the thinking window is a separate inner element).
+- `11_long_answers` (complete) — the untouched answer-bubble behavior.
+
+## Tasks
+1. `01_disable_thinking_scroll.md` — the CSS change + source-level unit
+   pins.
+2. `02_e2e_story_suite_commit.md` — `tests/e2e/test_thinking_no_scroll.py`
+   (the story gate, isolated), regression suites, story file, final
+   validation, the single atomic commit, phase move to `complete/`.
+
+## Locked decisions
+- **A11 untouched** — no CDN, pure CSS. **A16 honored** — one new story
+  E2E suite + adapted regressions. No anchor changed.
+
+## Testing & Quality
+- **Unit (source-level, new `tests/unit/test_thinking_no_scroll.py`,
+  repo source-pin pattern):** `styles.css` carries
+  `details.thinking .thinking-text` with `overflow-y: hidden` and
+  `max-height: 320px`; the phase-17 pin line
+  (`textEl.scrollTop = textEl.scrollHeight`) still present in `app.js`
+  (the live-tail mechanism must not be lost).
+- **Integration:** none (no `app/` changes).
+- **Coverage:** frontend-only; the >90% `app/` gate is unaffected.
+- **E2E:** `tests/e2e/test_thinking_no_scroll.py` (task 02), green
+  **in isolation** (prereq `podman compose up -d db`).
+- **Lint/types:** `uv run ruff check . && uv run pyright` clean.
+
+## Completion Criteria
+- [ ] With a long thinking stream, wheel/mouse-drag/keyboard on
+      `.thinking-text` do **not** move it; the visible content is always
+      the live tail (`scrollTop === scrollHeight` after each chunk,
+      within 1px).
+- [ ] Computed style: `overflow-y: hidden`, `max-height: 320px`.
+- [ ] A long **answer** bubble still scrolls normally; a restored
+      collapsed Thinking block still renders (phase 17 regression).
+- [ ] `uv run pytest` green; `uv run pytest --cov=app
+      --cov-report=term-missing` ≥ today's number.
+- [ ] `uv run pytest tests/e2e/test_thinking_no_scroll.py -v --no-cov`
+      green in isolation; regressions green in isolation:
+      `test_thinking_display.py`, `test_long_answers.py`.
+- [ ] `uv run ruff check . && uv run pyright` clean.
+- [ ] UI Structure Check (AGENTS.md rule 5): no new surface; the block
+      keeps its focus-visible summary, aria contract, and reduced-motion
+      behavior.
+- [ ] `.agents/user_stories/thinking-no-scroll.md` exists.
+- [ ] One `--no-gpg-sign` commit (below);
+      `.agents/phases/todo/21_thinking_no_scroll/` moved to
+      `.agents/phases/complete/`.
+
+## Commit
+```bash
+git add -A .agents/ frontend/ tests/ && git commit --no-gpg-sign -m "fix(ui): thinking window no longer scrolls — live 320px view pinned to the stream tail"
+```