Conflict recovery forks a copy of the transcript on every save conflict
(esengine#5993). The triggers are fixed, but every fork that ever happened still
sits in the session list until the user trashes it by hand — the
"会话列表被污染" half of that report was never addressed.

Add a conservative GC. agent.ReclaimableRecoveryBranches reclaims a
branch only when EVERY condition holds, because a recovery branch exists
precisely to prevent data loss:
  - meta says Recovered with a fork digest;
  - the transcript still matches that digest (never continued on — one
    follow-up turn disqualifies it permanently);
  - the parent (meta.ParentID) still exists and COVERS the branch content
    (equal digest, or branch is a compatible prefix — the same checks
    SaveRecoveryBranch uses to declare recovery unnecessary);
  - no runtime holds the branch's session lease;
  - it has been idle past a 24h grace period.

The desktop sweeps every session directory it lists from, once at startup
and every 6h, re-checking liveness (lease + open-in-tab) immediately
before disposal and routing removal through DeleteSession — so a swept
branch is moved to the recoverable session trash, never hard-deleted.

Verified the coverage guard by reverting it: a diverged fork (content
present nowhere else) then becomes reclaimable and the test fails.

Review catch: startup launches restoreOrBuildTabs on a goroutine and
started the GC immediately after, so the first sweep could run against
the pre-restore empty tab map. Two consequences: sessionOpenInAnyTab
would judge every saved tab's session as closed (a recovery tab about
to be restored could be swept to trash), and DeleteSession's tab-list
persistence would overwrite desktop-tabs.json with the empty snapshot,
wiping the user's saved tab layout.

Add a tabsRestored gate: armed in startup before the restore goroutine
launches, closed via defer when restoreOrBuildTabs finishes (both the
saved-tabs and first-launch paths, and the panic-recover path). The
GC's first sweep waits on the gate or ctx cancellation. markTabsRestored
is idempotent; tabsRestoredSignal reports already-restored when the gate
was never armed, so App instances built directly in tests keep working.

Regression test forces the exact race: gate closed → sweep provably
blocked and the branch untouched; restore lands the tab holding the
branch and opens the gate → the sweep runs and skips the now-open
branch.