Add Slack context source adapter

[mysubb01]

Summary

• add a Slack context-source adapter for messages from allowlisted channels via conversations.history
• require channel_ids so ingest never scans full workspace history, with max_messages_per_channel as a per-channel cap
• add Slack connection parsing, driver schema support, local pull-config wiring, docs, and focused tests

Verification

• pnpm --filter @kaelio/ktx exec vitest run src/context/connections/slack-config.test.ts src/context/ingest/adapters/slack/slack-client.test.ts src/context/ingest/adapters/slack/slack.adapter.test.ts src/context/project/driver-schemas.test.ts src/context/ingest/local-adapters.test.ts
• pnpm --filter @kaelio/ktx run type-check
• pnpm run dead-code
• pnpm --filter @kaelio/ktx run test
• pnpm --filter @kaelio/ktx run build

Closes #170

[vercel]

@loyal516 is attempting to deploy a commit to the Kaelio Team on Vercel.

A member of the Team first needs to authorize it.

[mysubb01]

Updated this PR to match the latest issue scope: it now ingests messages from configured allowlisted channels via conversations.history, keeps channel_ids required, and adds max_messages_per_channel as a per-channel cap. I also removed the canvas/pins-focused path from the implementation and docs.

Local verification after the update:

• targeted vitest: 5 files / 52 tests passed
• type-check passed
• dead-code passed
• package test: 338 files / 2388 tests passed
• build passed