docs: update existing pages, ACB theme, README features (Phase 3)

[bertheto]

Documentation Phase 3 — Update existing pages, ACB theme, README features

This PR completes the documentation effort started in PRs #42 (MkDocs site) and #43 (condensed README).

Axe 1 — Update existing MkDocs pages (accuracy fixes)

docs/reference/rest-api.md — 16 missing endpoints added:

• Messages: reactions (POST/DELETE/GET), edit (PUT), history (GET)
• Search (GET /api/search) and Metrics (GET /api/metrics)
• Threads: settings (GET/POST), admin (GET/POST decision), export (GET), sync-context (POST), agents (GET)
• Agents: kick (POST)

docs/reference/tools.md — corrections:

• msg_post: sync fields (expected_last_seq, reply_token) are required for MCP callers (not optional)
• msg_react/msg_unreact: agent_id is optional (not required)
• thread_list: documented cursor pagination (limit, before, next_cursor, envelope response)

docs/getting-started/config.md — 6 missing env vars added + corrected default DB path (source mode vs installed package mode)

docs/development/structure.md — added missing data models (MessageEdit, Reaction, ThreadSettings) + new Data Models section

Axe 3 — README improvements

• Added 8 missing features: bus_connect (one-step), message editing, reactions, FTS5 search, thread templates, admin coordinator, reply-to threading, agent skills
• Renamed "Zero external dependencies" to "No external infrastructure"
• Documentation link opens in a new browser tab (target="_blank")

Axe 4 — MkDocs theme aligned with ACB web UI

• OS theme auto-detection: media queries in mkdocs.yml — site follows system dark/light preference automatically
• docs/stylesheets/acb-theme.css:
  • Header: dark background #0a0d12 matching ACB --bg-base
  • Title: gradient linear-gradient(135deg, #3b82f6, #a855f7) matching ACB topbar logo
  • Nav tabs: #1e3a8a (dark) / #2563eb (light)
  • Section title on scroll: solid white (readable)
  • Code blocks: #111620 background matching ACB --bg-input
  • Copy button: same background as code block
  • Tables, admonitions, footer aligned with ACB palette

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Validated locally with mkdocs serve. No code changes — documentation only.