docs(guides): DOC-05 - add reactions guide

[bertheto]

Summary

• Add docs/guides/reactions.md: complete guide for the reactions system (UP-13)
• Register the guide in mkdocs.yml nav under Guides > Reactions

Guide contents (10 sections)

1. Introduction — lightweight free-form annotations on messages
2. Data Model — Reaction fields, UNIQUE constraint (message_id, agent_id, reaction)
3. Idempotency — INSERT OR IGNORE, silent duplicate handling, no-op msg_unreact
4. MCP Tools — msg_react / msg_unreact params, response examples, MCP vs REST agent_id note
5. REST API — POST, DELETE, GET endpoints with curl examples and error table
6. SSE Events — msg.react / msg.unreact payloads, no noise on no-ops
7. UI Rendering — pill grouping, count badge, agent tooltip, dark/light theme
8. Naming Conventions — recommended labels, emoji support, case-sensitivity warning
9. Bulk Loading — msg_reactions_bulk batch query, 3 auto-use points
10. Error Reference — MESSAGE_NOT_FOUND, empty reaction, DB timeout

Source references

• src/db/models.py L66-73
• src/db/crud.py L1509-1641
• src/main.py L928-998
• src/mcp_server.py L677-703
• src/tools/dispatch.py L855-891
• src/static/index.html L1272-1348
• src/static/css/main.css L1020-1043

Test plan

• mkdocs serve — verify Reactions appears in Guides nav and page renders without errors
• Check all admonitions render (note, tip, warning)
• Verify code blocks have correct syntax highlighting