fix: panel logo (#160), diff base fallback (#161), doc branch checkout (#162)

#160 — panel /roboco-logo.png "received null":
    next/image optimizer fails for static public assets in Next.js
    standalone mode. Added `unoptimized` to the sidebar logo Image so
    it serves the static file directly (validated on panel rebuild).

#161 — QA/doc evidence pr_diff_summary empty:
    A leaf dev branch's parent_branch_for is the cell-PM branch, which
    is never pushed (only devs push their leaf branch). diff against a
    non-existent origin/<parent> returned empty. Added
    GitService._resolve_diff_base + _default_branch_ref + _ref_exists:
    diff/list_changed_files fall back to the repo default branch
    (origin/HEAD → master/main) when origin/<parent> is absent.

#162 — claim_doc_task BRANCH_MISMATCH loop:
    The documenter's clone is separate from the dev's; the task branch
    already existed (dev created it) so no checkout ran in the doc
    workspace — roboco_docs_write / commit failed BRANCH_MISMATCH and
    the doc looped. Fixes:
    (a) new GitService.checkout_branch_in_agent_workspace; claim_doc_task
        checks out the task branch into the doc clone (best-effort —
        a checkout hiccup never fails the claim).
    (b) BRANCH_MISMATCH remediate now lists all four role claim verbs
        (i_will_work_on / i_will_plan / claim_doc_task / claim_review).
    (d) give_me_work next-hint is role+status aware via _claim_verb_hint
        (doc→claim_doc_task, qa→claim_review, pm→i_will_plan, else dev).
    Facet (c) (i_am_blocked "Not Found" for doc) was only reachable via
    the stuck-without-checkout path; primary fix removes it.

Smoke-11 reached dev→QA→doc (deepest ever) and validated the prior
6 fixes (panel flood gone, #158/#159/#157 confirmed). These three
clear the doc-phase blockers found in that run.

tests/unit/services/test_git_diff_base_fallback.py

@@ -0,0 +1,90 @@
+"""Task #161: GitService diff base falls back to default branch.
+
+A leaf dev branch's parent (per parent_branch_for) is the cell-PM
+branch feature/{team}/{root}--{cellpm}, which is NEVER pushed — only
+devs push their own leaf branch. Diffing against a non-existent
+origin/<parent> returns an empty diff, so QA / docs saw nothing.
+_resolve_diff_base must fall back to the repo default branch when the
+parent ref is absent on origin.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Any
+from unittest.mock import AsyncMock
+
+import pytest
+from roboco.services.git import GitService
+
+
+def _git_service() -> GitService:
+    return GitService.__new__(GitService)
+
+
+@pytest.mark.asyncio
+async def test_resolve_diff_base_uses_parent_when_pushed() -> None:
+    """When origin/<parent> exists, use it (normal case)."""
+    svc = _git_service()
+    svc._run_git = AsyncMock()  # type: ignore[method-assign]
+    svc._ref_exists = AsyncMock(return_value=True)  # type: ignore[method-assign]
+    ws = Path("/tmp/ws")
+
+    base = await svc._resolve_diff_base(
+        ws, "feature/backend/root1234--cellpm56--dev78901"
+    )
+    # parent_branch_for strips the last --segment.
+    assert base == "origin/feature/backend/root1234--cellpm56"
+
+
+@pytest.mark.asyncio
+async def test_resolve_diff_base_falls_back_when_parent_absent() -> None:
+    """When origin/<parent> does NOT exist (cell-PM branch never pushed),
+    fall back to the repo default branch via origin/HEAD."""
+    svc = _git_service()
+    svc._run_git = AsyncMock()  # type: ignore[method-assign]
+    # parent ref absent → _ref_exists False for the parent check.
+    svc._ref_exists = AsyncMock(return_value=False)  # type: ignore[method-assign]
+    svc._default_branch_ref = AsyncMock(  # type: ignore[method-assign]
+        return_value="origin/master"
+    )
+    ws = Path("/tmp/ws")
+
+    base = await svc._resolve_diff_base(
+        ws, "feature/backend/root1234--cellpm56--dev78901"
+    )
+    assert base == "origin/master"
+    svc._default_branch_ref.assert_awaited_once()
+
+
+@pytest.mark.asyncio
+async def test_default_branch_ref_prefers_origin_head() -> None:
+    """origin/HEAD symbolic-ref is the canonical default-branch pointer."""
+    svc = _git_service()
+
+    async def fake_run(_ws: Any, args: list[str], **_kw: Any) -> Any:
+        if args[:2] == ["symbolic-ref", "--quiet"]:
+            return type(
+                "R", (), {"returncode": 0, "stdout": "refs/remotes/origin/main\n"}
+            )()
+        return type("R", (), {"returncode": 1, "stdout": ""})()
+
+    svc._run_git = fake_run  # type: ignore[method-assign]
+    ref = await svc._default_branch_ref(Path("/tmp/ws"))
+    assert ref == "origin/main"
+
+
+@pytest.mark.asyncio
+async def test_default_branch_ref_fallback_when_no_head() -> None:
+    """No origin/HEAD → probe origin/master then origin/main; final
+    hard fallback is origin/master so the git invocation stays valid."""
+    svc = _git_service()
+
+    async def fake_run(_ws: Any, _args: list[str], **_kw: Any) -> Any:
+        # symbolic-ref fails; fetches succeed but ref never verifies.
+        return type("R", (), {"returncode": 1, "stdout": ""})()
+
+    svc._run_git = fake_run  # type: ignore[method-assign]
+    svc._ref_exists = AsyncMock(return_value=False)  # type: ignore[method-assign]
+    ref = await svc._default_branch_ref(Path("/tmp/ws"))
+    assert ref == "origin/master"