fix: don't discard codex/gemini/antigravity conversations on Resume; fix DELETE ownership dup + missing broadcast

resumeHistorySession() creates the resumed row in its own mode via a
modeConfigKey map (opencode/pi/grok/omp -> continueSession, deepseek ->
resumeSession) and retires the old row afterward. codex, gemini and
antigravity were missing from that map, so resuming one of their rows
started a brand-new session with NO continuation while still deleting
the row it came from -- silent data loss dressed as the duplicate-row
fix. Gate row retirement on continuesSomething (true only for modes that
actually got a continuation config) instead of wiring an unverified
sessionId->native-conversation-id assumption for the three affected CLIs.

DELETE /api/sessions/:id reimplemented the ownership 404 check inline in
two places instead of going through findSessionOrFail, and its
persisted-only-session branch never broadcast session:deleted, so other
open tabs kept the retired row until their next unrelated fetch. Extract
the shared 404 into sessionNotFoundError(), add findPersistedSessionOrFail()
alongside findSessionOrFail() in route-helpers.ts (same ownership
contract, returns a SessionState instead of a live Session), and use both
from the route instead of inline checks. Add the missing broadcast.

test/routes/session-routes.test.ts

@@ -356,6 +356,10 @@ describe('session-routes', () => {
       expect(body.success).toBe(true);
       expect(harness.ctx.store.demoteOrRemoveSession).toHaveBeenCalledWith('ghost-session');
       expect(harness.ctx.cleanupSession).not.toHaveBeenCalled();
+      // Ark0N/Codeman#353 review: the persisted-only branch used to demote/remove
+      // with no broadcast, so other open tabs kept showing the retired row until
+      // their next unrelated fetch.
+      expect(harness.ctx.broadcast).toHaveBeenCalledWith('session:deleted', { id: 'ghost-session' });
     });
 
     it('404s a persisted-only session id the state store does not recognize either', async () => {