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/user_stories/sources-midstream.md

@@ -0,0 +1,73 @@
+# Story: Sources Mid-Stream — an in-flight answer is not lost on navigation
+
+**Phase:** `20_sources_midstream_bug` · **E2E:** `tests/e2e/test_sources_midstream_bug.py`
+
+## Bug report (TODO.md L3, verbatim)
+
+> *"Clicking "sources" while chat is generating clears chat and result
+> will never show up"*
+
+## Narrative
+
+As **a user with a question in flight**, I want to **click "Sources"
+while Brain is still generating** without losing the answer — when I
+come back to the chat, what had already streamed should be waiting for
+me.
+
+- **Given** I asked a question and Brain's answer is still streaming
+- **When** I click the "Sources" nav link (or otherwise leave the chat
+  page) mid-stream
+- **Then** the stream is aborted by the navigation (no server-side
+  resume — A10 stays stateless), but returning to `/` shows my question
+  **and** the partial answer that had already streamed, rendered exactly
+  like any brain message (thinking block included, if any had streamed).
+
+## Owner-confirmed (2026-08-24, roadmap A1)
+1. **A partial answer is persisted as a plain brain message** — no
+   "(partial)" marker, no sources/suggestions (the turn is dead; the
+   user can re-ask for the full answer).
+2. Navigation **before the first answer token** (pure thinking) persists
+   nothing brain-side: the question is restored, no empty/partial
+   bubble.
+3. The deliberate **New Chat** `clearChatStorage()` (sources/viewer
+   pages) is untouched — that clear is by design (phase 14/19).
+4. No server-side resume (A10 stays stateless) and no "leave page?"
+   confirmation dialog.
+
+## Acceptance criteria
+1. `pagehide` (navigate-away / bfcache store) with an in-flight turn
+   (`thinking` or `streaming`) and non-empty streamed text persists the
+   partial raw answer via the existing `rememberBrainTurn` save-point
+   helper (idempotent per turn — `persistedOnLeave`).
+2. Returning to `/` restores the question and the partial brain message
+   through the existing restore path (escape-first markdown, optional
+   collapsed thinking block) — no new UI surface, no error banner.
+3. Navigation during pure thinking (no answer tokens) persists nothing
+   brain-side: exactly the user message is restored.
+4. A completed turn persists exactly as before (done metadata intact;
+   no duplicate brain message).
+5. New Chat from the sources page still clears the conversation
+   (`clearChatStorage`, phase 14/19 — untouched).
+6. Unit + integration green, `app/` coverage ≥ today's number (>90%
+   gate), story E2E green in isolation, one `--no-gpg-sign` commit.
+
+## Playwright Mapping Rule
+**Test Scenario → `tests/e2e/test_sources_midstream_bug.py`** (mock LLM,
+seeded KB):
+1. `test_partial_answer_survives_sources_nav_midstream` — admin (login,
+   `next=/`), send the ~9s long-answer question, wait for the first
+   streamed frames to render, click `#nav-sources` (lands on
+   `/sources.html`), `page.goto("/")` → question + partial answer
+   rendered, no `role="alert"`, `bor.chat.v1` holds the partial brain
+   message (starts with the first streamed chunk, shorter than the full
+   answer, no done metadata).
+2. `test_no_orphan_brain_message_when_navigated_before_first_token` —
+   the mock streams the phase-17 thinking scratchpad, then a 4s
+   pre-token pause; navigate during the pause, return to `/` → exactly
+   one user message, zero brain messages, in the DOM and in storage.
+3. `test_completed_turn_unaffected` — a fully settled turn → trip to
+   Sources and back → full answer, source chip, and a byte-identical
+   `bor.chat.v1` payload (done metadata intact, no duplicate).
+4. `test_new_chat_still_clears_conversation` — completed turn →
+   `/sources.html` → click the sources-page New Chat button → lands on
+   `/` with the empty state and `bor.chat.v1` removed.