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/header-consistency.md

@@ -0,0 +1,38 @@
+# Story: One Header, Same Size Everywhere
+
+**Phase:** `12_header_consistency.md` · **E2E:** `tests/e2e/test_header_consistency.py`
+
+## Narrative
+
+As **a user**, I want the top bar to be the **same height on every page**
+so moving between Chat, Sources, and a document never feels like the
+app "grew".
+
+Today the document viewer (the page whose back button says "Sources")
+uses a content-sized header (≈66px desktop / ≈63px+ mobile) while Chat
+and Sources use the fixed 64px bar (58px mobile) — the perceived
+"header changes size between sources and chat".
+
+- **Given** any of the three pages at any viewport
+- **When** I look at the sticky top bar
+- **Then** it is exactly `var(--header-h)` tall on all of them.
+
+## Acceptance criteria
+1. `.doc-header` is exactly `var(--header-h)` tall (64px desktop,
+   58px ≤640px) — same box as `.app-header` on `/` and `/sources.html`.
+2. Title + meta still fit: title stays single-line (ellipsis), meta row
+   clips instead of growing the header on narrow screens.
+3. Existing phase-10 viewer behavior (back link, title, meta badges,
+   a11y frame) unchanged.
+4. Unit + integration green, `app/` coverage >90%, story E2E green in
+   isolation, one `--no-gpg-sign` commit.
+
+## Playwright Mapping Rule
+**Test Scenario → `tests/e2e/test_header_consistency.py`**:
+1. `test_header_height_identical_across_pages_desktop` — measured
+   bounding boxes of `.app-header` (chat), `.app-header` (sources),
+   `.doc-header` (viewer) are all exactly 64px tall.
+2. `test_header_height_identical_across_pages_mobile` — same at a 375px
+   viewport: all exactly 58px.
+3. `test_viewer_header_content_still_fits` — title ellipsizes, meta row
+   visible, back link + badges present (regression guard for phase 10).