feat(server): add Slack message sending

[siisee11]

Summary

• advertise Slack send_message only when the connected bot has the chat:write scope
• normalize and execute message sends through Slack chat.postMessage
• validate text, blocks, and optional thread timestamps and document the required scope in provider guidance

Validation

• bun run format
• mise exec -- bunx oxlint packages/db/src/source-providers.ts packages/server/src/source-api/adapters/slack.ts packages/server/src/source-api/adapters/slack.test.ts --type-aware --type-check --format json
• bunx turbo test --json --filter=@onequery/server — 49 files, 190 tests passed
• mise exec -- bunx turbo test --json --filter=@onequery/server -- src/source-api/adapters/slack.test.ts — 8 tests passed

Known baseline issue

• bunx turbo typecheck --json --filter=@onequery/server --filter=@onequery/db remains blocked by existing Buffer compatibility errors in unrelated packages/db/src/pglite.ts and server audit/continuation files; none of the reported errors are in this change

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	onequery-landing	43c3898	Commit Preview URL Branch Preview URL	Jul 15 2026, 02:13 AM