(orchestration): Hydrate task context from Slack thread URLs

[krokoko]

Component

API or orchestration

Describe the feature

Allow task creation (API, CLI, and Slack-origin flows) to accept a Slack thread permalink and hydrate the prompt from the full thread: messages, linked files, and bot replies in chronological order, within the existing hydration token budget.

Use case

Teams often spec work in Slack: discussion, clarifications, and links accumulate in a thread before anyone files a ticket. Submitting only a one-line description drops that context. Background agents work best when the collaboration artifact (the thread) is the spec.

Proposed solution

1. Extend create-task schema with optional slack_thread_url (or channel-specific metadata when origin is already Slack).
2. Add hydration fetcher using the installed Slack app token (same credentials as notification dispatcher).
3. Normalize thread to markdown sections; trim oldest non-essential messages first (mirror PR comment trimming).
4. Screen fetched text through existing Guardrails / sanitization paths.
5. CLI: bgagent submit --slack-thread <url> ....

Other information

• Requires linked Slack workspace (existing setup guide).
• Complements shipped Slack @mention submission; this targets richer context for the same channel.
• Consider rate limits and channel:history scopes in setup docs.

Acknowledgements

• I may be able to implement this feature
• This might be a breaking change

Acceptance criteria

• API and CLI accept optional Slack thread reference; types mirrored in cli/src/types.ts.
• Hydration includes thread body in prompt when provided; title/issue body still preserved.
• Token budget enforcement matches existing hydration rules.
• SSRF-safe: only slack.com / workspace URLs from configured workspace; reject arbitrary URLs.
• Tests use mocked Slack API responses in cdk/test/handlers/shared/context-hydration.test.ts.