Add workspace-wide Slack message search

[odfalik]

Summary

• add a search_messages MCP tool backed by Slack's native search.messages endpoint (no recent-history scanning)
• accept native Slack search syntax plus bounded result pages, sorting, and bounded threaded context
• return UTC and Slack timestamps, human channel/DM labels, channel IDs, authors, message text, permalinks, and actionable get_thread arguments
• keep bot-token writes/reactions and existing user-token read behavior unchanged; use a dedicated user-token-only search client
• add focused search/auth/pagination/thread-context tests and make CI run the test suite on Python 3.10 and 3.12
• replace stale Socket Mode docs with the current outbound-only architecture and exact OAuth setup

Slack API and authorization

Slack restricts search.messages to user OAuth tokens with the legacy user scope search:read. Bot xoxb tokens return not_allowed_token_type; user tokens without the scope return missing_scope. The tool surfaces actionable messages for both cases.

The currently configured SLACK_USER_TOKEN was checked against the real API and returns missing_scope with needed=search:read. No fallback or recent-history approximation is used.

One-time reauthorization

1. Open https://api.slack.com/apps and select the Slack app.
2. Open OAuth & Permissions.
3. Under Scopes → User Token Scopes, add search:read.
4. Click Reinstall to Workspace and approve the new permission.
5. Copy the resulting User OAuth Token (xoxp-...), not the Bot User OAuth Token.
6. Set SLACK_USER_TOKEN=xoxp-... in ~/.claude/channels/slack-channel/.env.
7. Restart Claude Code or the MCP host so the server reloads credentials.

Validation

• uv run --python 3.10 --extra dev python -m pytest -q — 19 passed
• uv run --python 3.12 --extra dev python -m pytest -q — 19 passed
• uv run --extra dev ruff check slack_channel/ tests/ — passed
• uv build — passed
• git diff --check — passed