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/phases/complete/12_header_consistency/00_phase.md

@@ -0,0 +1,47 @@
+# Phase 12 — One Header, Same Size Everywhere
+
+**Story:** `.agent/user_stories/header-consistency.md`
+**Context:** owner report 2026-08-22 — "the header changes size between
+sources and chat."
+
+## Goal
+The sticky top bar is the **same height on every page**: Chat, Sources,
+and the document viewer (the page whose back button says "Sources" — the
+one users compare against chat).
+
+## Diagnosis (measured 2026-08-22, headless Chromium)
+- Desktop: `.app-header` = 64px on `/` **and** `/sources.html`
+  (identical markup/CSS); `.doc-header` = **66.375px** (content-sized:
+  `padding-block: 0.7rem` around a 44px back pill + 2-line title block).
+- Mobile (≤640px): app header 58px; `.doc-header` = **63.19px** (and
+  grows further when the title/meta wrap).
+So the "sources header" (viewer back button = "Sources") is visibly
+taller than the chat header — on desktop and mobile.
+
+## Implementation steps
+1. **CSS** (`frontend/assets/styles.css`):
+   - `.doc-header { height: var(--header-h); }` — same fixed box as
+     `.app-header`.
+   - `.doc-header-inner { height: 100%; padding-block: 0; }` (center
+     vertically; the 44px pill + title/meta block fit in 64/58px).
+   - Mobile: replace the wrapping rule with `flex-wrap: nowrap`;
+     `.doc-meta { flex-wrap: nowrap; overflow: hidden; }` so the meta
+     row clips (path already ellipsizes) instead of growing the header.
+     `#doc-title` already ellipsizes on one line.
+2. No HTML/JS changes (ids/classes unchanged → phase-10 tests intact).
+3. **PLAN §7.1:** note the shared 64px header bar across all three pages.
+
+## Locked decisions
+None. A11/Phase-08 tokens untouched (same colors, same hairline).
+
+## Testing & Quality
+- E2E: `tests/e2e/test_header_consistency.py` per the story mapping
+  (desktop 1280px + mobile 375px: three pages, three identical boxes).
+- Regression: `test_document_viewer.py` green in isolation (viewer
+  content/meta/back link unchanged).
+- Coverage gate unchanged (no `app/` code touched → stays >90%).
+
+## Commit
+```bash
+git add -A .agent/ frontend/assets/styles.css tests/e2e/test_header_consistency.py && git commit --no-gpg-sign -m "fix(ui): uniform header bar height on chat, sources, and the document viewer"
+```