Split remote.ts by topic, starting with migrations

[EhabY]

`src/remote/remote.ts` is 1,207 lines and mixes every concern of remote setup in one class. Recent work (per-editor SSH configs, legacy authority migration) keeps growing it.

Topics currently mixed into the class

• Setup orchestration: `setup`, `setupCoderRemote`
• Migrations: `migrateLegacyAuthority` (authority), `migrateToSecretsStorage`, `migrateSessionTokenFile`, `migrateSessionAuthFromFiles` (credentials)
• SSH config assembly: `updateSSHConfig`, `writeRemoteSshConfig`, `buildProxyCommand`, `getSshConfigPath`, `getLogArgs`, `getLogDir`
• Workspace lifecycle: `lookupWorkspace`, `waitForWorkspaceReady`, `resolveAgent`
• Auth/session: `ensureLoggedInAndRetry`, `showSessionExpiredDialog`, `watchRemoteSessionAuth`
• Binary management: `resolveRemoteBinary`, `checkCompatibility`
• Window/UI glue: `createAgentMetadataStatusBar`, `watchSettings`, `registerLabelFormatter`, `closeRemote`, `reloadWindow`

Proposal

1. Extract migrations first, e.g. into `src/remote/migrations.ts`. They are self-contained, interact little with the rest of setup, and each has a deletion horizon; keeping them together makes them easy to find and eventually remove.
2. If that lands well, extract SSH config assembly next. It is the most test-heavy area and only needs the path resolver, settings, and logger.
3. Keep orchestration (`setup`/`setupCoderRemote`) in `remote.ts`.

Non-goals: no behavior changes and no new abstractions; move plain methods by topic and keep the telemetry span threading as is.

Filed by Claude Code for @EhabY.

[linear-code]

ENG-3137