FN-5838: suppress stale integration-advance action prompts

Refine merge-advance status handling so Git Manager only requests action when the checkout is truly behind.

- classify recent integration advances as reachable, orphaned, subsumed, or pending before marking them actionable
- detect subsumed advances by comparing commit patch fingerprints from recent HEAD history
- expose advance resolution metadata in API status payloads and route exports
- update Git Manager UI to count only pending actionable advances, gate Sync Working Tree visibility, and allow dismissing handled orphaned/subsumed rows
- expand dashboard tests and docs to cover stale false-positive scenarios and new resolution behavior

Files changed:
 docs/dashboard-guide.md                            |  6 ++
 packages/dashboard/app/api/legacy.ts               |  1 +
 .../dashboard/app/components/GitManagerModal.tsx   | 45 ++++++----
 .../components/__tests__/GitManagerModal.test.tsx  | 63 ++++++++++++++
 .../dashboard/src/__tests__/routes-git.test.ts     | 97 +++++++++++++++++++++-
 .../dashboard/src/routes/register-git-github.ts    | 94 ++++++++++++++-------
 6 files changed, 259 insertions(+), 47 deletions(-)

Fusion-Task-Id: FN-5838

Fusion-Task-Lineage: 65d1317a-7618-4fff-9875-c845474888ed

docs/dashboard-guide.md

@@ -261,6 +261,12 @@ How to react:
 - Click **Pull** to run Smart Pull (`POST /api/git/smart-pull`), including the stash-conflict flow in `StashConflictModal` when needed
 - Use the dismiss close button to hide the notice
 - Treat dirty/untracked warnings as a hint that local changes may be auto-stashed during pull
+- In Git Manager's "Recent integration-branch advances" panel, entries are classified as `pending`, `reachable`, `subsumed`, or `orphaned`:
+  - `pending`: actionable (not reflected in HEAD; Sync can help)
+  - `reachable`: commit already reachable from HEAD
+  - `subsumed`: equivalent patch content already landed under a different SHA (history rewrite/re-squash)
+  - `orphaned`: recorded SHA no longer exists locally after history rewrite
+- **Sync working tree** is shown only when there is at least one `pending` entry and HEAD is not already aligned with the integration tip; handled entries can be dismissed from the panel.
 
 Push follow-up (when shown):
 - If the integration branch is ahead of `origin`, the banner can show push controls with ahead count