FN-5839: prevent stale integration-advance sync prompts

Treat rewritten integration-branch advances as handled when the working tree is already aligned.

- Add a new `superseded` resolution for unreachable advance SHAs that remain after history rewrites while HEAD matches the local integration tip.
- Update Git status collection logic and route/API status typings to propagate the new resolution state.
- Update Git Manager modal messaging and dismiss behavior so handled `superseded` entries do not show a sync CTA.
- Extend dashboard route and modal tests, and refresh dashboard guide docs for the new classification behavior.

Files changed:
 docs/dashboard-guide.md                            |  5 +--
 packages/dashboard/app/api/legacy.ts               |  2 +-
 .../dashboard/app/components/GitManagerModal.tsx   |  6 ++--
 .../components/__tests__/GitManagerModal.test.tsx  |  5 ++-
 .../dashboard/src/__tests__/routes-git.test.ts     | 36 ++++++++++++++++++++--
 .../dashboard/src/routes/register-git-github.ts    | 13 ++++++--
 6 files changed, 55 insertions(+), 12 deletions(-)

Fusion-Task-Id: FN-5839

Fusion-Task-Lineage: 6b4afb73-587d-4319-8ad8-f1d5d54bc7bf

packages/dashboard/app/api/legacy.ts

@@ -2708,7 +2708,7 @@ export interface GitStatus {
     advancedAt: string;
     autoSyncOutcome?: string;
     needsAction: boolean;
-    resolution: "reachable" | "orphaned" | "subsumed" | "pending";
+    resolution: "reachable" | "orphaned" | "subsumed" | "superseded" | "pending";
   }>;
 }