projects: refresh the Slack directory on demand when a channel link misses (#487)

Linking a just-created Slack channel to a project failed with 'channel not found among the channels you can see' because the check runs against the synced directory copy, which only refreshes on timers and a few membership events — a brand-new channel isn't there yet. On a channel-link miss, core now parks a syncDirectory surface-context request; the Slack surface fulfils it with a forced directory sync and the lookup retries once before failing. The Slack surface also refreshes proactively on channel_created, channel_rename, and channel_unarchive events, which are added to the app manifest.

**Deployment notes**

Release-note: re-apply the Slack app manifest to enable proactive channel-lifecycle refresh
Slack app manifest gains channel_created / channel_rename / channel_unarchive bot events —
the proactive directory refresh only works after an org re-applies its Slack app manifest; the on-
demand syncDirectory retry path works without it, so nothing regresses if they don't
New surface-context request type syncDirectory: in the blue-green window a new core parking
the request against an old Slack surface just goes unfulfilled and the lookup fails exactly as it did
pre-change (no worse)
No DB or config changes

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