fix(omp): retire the old row on resume, and let DELETE remove persisted-only sessions

Every non-claude "Resume" click creates a brand-new Codeman session
(there is no id to reattach to), but the old row was never cleaned up
-- click resume on the same conversation a few times and the session
list fills up with duplicate rows sharing one name. resumeHistorySession
now retires the row it resumed from after the new one starts.

That retirement needs DELETE to actually work on a row that was never
live in the first place (the normal case for anything showing up in
"Resume Conversation"): findSessionOrFail only checks the in-memory
live-session map, so DELETE 404s on a persisted-only entry today. Give
the route a fallback: when the id isn't live, look it up in persisted
state instead and demote/remove it there (respecting the existing
pinned-session protection). Verified live against a real persisted-only
row via the API, and added route-test coverage for both the success
and still-truly-unknown-id cases (which needed a demoteOrRemoveSession
mock the route harness didn't have).

Also includes an unrelated pre-existing prettier drift fix picked up
by npm run format (omp-cli-resolver.ts, antigravity/opencode import
wrapping in session-routes.ts).

src/utils/omp-cli-resolver.ts

@@ -76,7 +76,11 @@ function probeOmpVersion(binPath: string): string | null {
 
 type OmpVersionProbe = (binPath: string) => string | null;
 
-function createOmpResolver(host?: CliResolverHost, versionProbe: OmpVersionProbe = probeOmpVersion, now?: () => number) {
+function createOmpResolver(
+  host?: CliResolverHost,
+  versionProbe: OmpVersionProbe = probeOmpVersion,
+  now?: () => number
+) {
   return createCliExecutableResolver<string>(
     {
       binary: 'omp',