feat(slack): support shared GitHub Cloud Agent workflows

[pandemicsyn]

Summary

• allow an already-connected Slack integration to remain compatible when its Kilo owner enters shared-GitHub mode
• aggregate cached repositories from every healthy tenant-owned GitHub association with exact association provenance
• revalidate the selected association for the Slack owner and forward its exact githubIntegrationId to Cloud Agent

Scope

• Slack must already be connected before sharing
• PR2 provider OAuth attempts continue to fence Slack setup against sharing
• Linear and Discord remain incompatible
• no Slack OAuth, credential, Chat SDK, webhook, uninstall, reconnect, Enterprise, schema, or migration changes
• no synthetic all-access repository fallback

Validation

• 22 focused tests, repeated twice after final fixture cleanup
• healthy/unhealthy sibling inventory and exact tenant provenance
• active Slack compatibility plus Linear/Discord and live-attempt fences
• web typecheck and lint
• formatting and diff checks
• exact final inventory: 3 production files, 3 test files, no migration