chore(agent): track .agent/ planning tree in git

Remove the blanket .agent/ gitignore so the phase roadmap, user
stories, reports, and PLAN.md are versioned with the code. Only
runtime artifacts (.agent/phase-sessions/, .agent/pipeline.log)
remain ignored. Update AGENTS.md git protocol rule to match.

.agent/user_stories/steering-notes.md

@@ -0,0 +1,49 @@
+# Story: Tune How Brain Answers (Steering Notes)
+
+**Phase:** `15_steering_notes.md` · **E2E:** `tests/e2e/test_steering.py`
+
+## Narrative
+
+As **a user**, when an answer **isn't quite right** (too chatty, wrong
+assumption, missing context), I want to **tune** Brain right there: a
+short instruction ("be more concise", "assume I'm on NixOS") that is
+**stored in the database** and **read into the system prompt** so every
+future reply follows it.
+
+- **Given** an answer I don't like
+- **When** I click "Tune" under it and save an instruction
+- **Then** the note is persisted in Postgres, shown in the tuning
+  panel (where it can be removed), and injected into the system prompt
+  of every subsequent turn — verifiable in the answer.
+
+## Acceptance criteria
+1. New `steering_notes` table (id, note, created_at) via Alembic
+   migration 0003.
+2. Stateless API: `GET /api/steering` (newest first),
+   `POST /api/steering` (1–2000 chars, 201 / 422),
+   `DELETE /api/steering/{id}` (204 / 404).
+3. System prompt: when notes exist, a `<tuning>` section (numbered notes,
+   char-budgeted) is appended to **both** HIGH and DEFLECT prompts; when
+   none exist, the prompt is byte-identical to today's.
+4. Per-turn log line gains `tuning=N`.
+5. UI: "Tune" button under every completed brain answer (deflected too)
+   → inline form → save → confirmation; header "Tuning" toggle lists
+   notes with per-note delete; all labeled, live regions, ≥44px targets,
+   Phase-08 tokens.
+6. Notes render as text (XSS-safe).
+7. Unit + integration green, `app/` coverage >90%, story E2E green in
+   isolation, one `--no-gpg-sign` commit.
+
+## Playwright Mapping Rule
+**Test Scenario → `tests/e2e/test_steering.py`** (mock LLM, seeded KB —
+the mock echoes the first tuning note into its answer when `<tuning>` is
+present, so prompt injection is observable in the UI):
+1. `test_tune_under_answer_persists_and_steers` — answer → Tune → note
+   "STEEER-MARKER be concise" → saved → next answer contains the marker
+   (note reached the system prompt) → note visible in panel with count.
+2. `test_delete_note_stops_steering` — delete from panel → count 0 →
+   next answer has no marker.
+3. `test_note_rendered_as_text_xss_safe` — note with `<script>` renders
+   as visible escaped text, never executes.
+4. `test_tuning_panel_a11y` — toggle aria-expanded, region label,
+   delete buttons labeled, live region announces save/delete.