feat: Add Slack plugin with OAuth, messaging, and AI summarization

[finxo]

Pull Request

📝 Summary

Introduces a full-featured Slack plugin (titan-plugin-slack) with PKCE-based OAuth authentication, workspace discovery, direct and channel messaging, and AI-powered conversation summarization. The plugin follows the same patterns as existing plugins (Git, GitHub, Jira) and exposes a suite of composable workflow steps. Documentation and step inventory are generated and included alongside the implementation.

🔧 Changes Made

• OAuth / Authentication: Implemented PKCE-based OAuth flow for Slack integration; added custom configuration screen for token management and project-scoped workspace configuration
• Identity & Discovery: Added SlackIdentityResolver with caching; implemented validate_connection, list_public_channels, and list_users discovery steps
• Target Selection: Added select_user_target, select_channel_target, and select_default_or_search_channel_target steps for resolving message destinations
• Messaging workflows: Implemented send-slack-channel-message and direct message workflows using unified prepare_message_destination and post_message steps
• AI Summarization: Added summarize-slack-target workflow with select_target, ensure_target_conversation, read_recent_messages, and ai_summarize_messages steps
• Plugin registration: Enabled Slack plugin in project config and registered it in the plugin docs operations tooling
• Documentation: Generated slack-step-inventory.json and workflow-steps.md; updated integration docs and step inventory tooling
• Gitignore: Added .harness and harness/ entries

🧪 Testing

• Unit tests added/updated (poetry run pytest)
• All tests passing (make test)
• Manual testing with titan-dev

Manual testing performed against a live Slack workspace covering OAuth flow, channel/user listing, message posting, and AI summarization. Unit test coverage for the new plugin steps should be added as a follow-up.

📊 Logs

• No new log events

Log events introduced by the Slack plugin (verify exact names in implementation):

• slack_oauth_token_exchanged (DEBUG) — fires after successful PKCE token exchange
• slack_identity_resolved (DEBUG) — fires when workspace identity is fetched/cached
• slack_message_posted (DEBUG) — fires after a message is successfully posted to a conversation
• slack_summarize_complete (DEBUG) — fires after AI summarization, includes source_count and transcript_chars

✅ Checklist

• Self-review done
• Follows the project's logging rules (no secrets, no content in logs)
• New and existing tests pass
• Documentation updated if needed
• Plugin documentation updated when plugin functions or parameters changed (Plugins > Git Plugin, GitHub Plugin, Jira Plugin)