create blink manuals

- Blink manual command in CLI (blink manual )
- Description: Implement CLI to list manuals and print Markdown for Slack, GitHub; fallback to list on invalid name
- Acceptance:
- blink manual shows available manuals
- blink manual slack prints Slack manual (Markdown)
- blink manual github prints GitHub manual (Markdown)
- Output consumable by LLMs and humans
- Replace injected Agents.md with manual placeholders
- Description: Remove large Agents MD injection; reference blink manual instead
- Acceptance:
- Edit mode uses placeholders
- Docs updated; no oversized prompt/context
- Author initial manuals
- Slack manual: auth, events, commands, posting, SDK patterns
- GitHub manual: auth, webhooks, PR review flow, comments, statuses
- Acceptance: Examples included; copy reviewed; versioned in repo