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.

panel/src/components/layout/sidebar.tsx

@@ -72,6 +72,7 @@ export function Sidebar() {
               width={32}
               height={32}
               priority
+              unoptimized
               className="h-8 w-8 rounded"
             />
             <span className="font-semibold text-lg">RoboCo</span>