feat(merger): rebase task branch onto remote before merge

Adds a new worktree setting that fetches the configured remote and rebases
the task branch onto the latest default-branch tip before the merger attempts
to merge it back. Catches concurrent pushes from other collaborators or
fusion workers on other hosts before they surface as merge conflicts —
anything the rebase can't fast-forward flows into the existing smart/AI
resolve pipeline (attempts 1–3) rather than needing new handling.

- `settings.worktreeRebaseBeforeMerge` (bool, default true) — gates the step.
- `settings.worktreeRebaseRemote` (string, default "") — which remote to
  fetch; empty falls back to git's configured remote for the default branch,
  then to the sole remote if there's only one, then to "origin".
- Rebase runs inside the task's worktree; failure aborts and falls through
  to the merge cascade. Rebase errors are warn-logged but never throw.
- Dashboard SettingsModal Worktrees section now has a toggle for the setting
  plus a remote dropdown populated from `/api/git/remotes/detailed`. The
  dropdown defaults to "Use git default" so no explicit selection is needed
  on first configure.

Also aligns the Last/Next heartbeat spans on the agent list card — both now
share the `.agent-heartbeat-last, .agent-heartbeat-next, .agent-heartbeat-saving`
font-size rule with a consistent line-height and inline-flex alignment so
the labels don't drift vertically when they share a row.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/src/settings-schema.ts

@@ -96,6 +96,8 @@ export const DEFAULT_PROJECT_SETTINGS = {
   defaultPresetBySize: {},
   autoResolveConflicts: true,
   smartConflictResolution: true,
+  worktreeRebaseBeforeMerge: true,
+  worktreeRebaseRemote: "",
   strictScopeEnforcement: false,
   buildRetryCount: 0,
   verificationFixRetries: 1,