Add Slack platform adapter and comprehensive platform skills system

Major Features:
- Implemented full Slack channel-bot adapter with Socket Mode
- Added 35+ Mastodon platform skills across 7 categories
- Created constitution publishing system for Mastodon

Slack Adapter (NEW):
- Full PlatformAdapter implementation (1071 lines)
- Socket Mode for real-time events
- 16 channel-scoped governance skills
- User group management as channel "roles"
- Channel access control and management
- Message pinning and moderation
- Platform limitations documentation
- Comprehensive setup guide (SLACK_SETUP.md)

Mastodon Platform Skills (ENHANCED):
- Account Moderation (9 skills): suspend, silence, disable, mark sensitive, delete status
- Account Management (4 skills): approve, reject, delete data, create account
- Report Management (4 skills): assign, unassign, resolve, reopen
- Federation Management (4 skills): block/unblock domains, allow/disallow federation
- Security Management (4 skills): block IP/email domains
- Constitution Management (2 skills): publish constitution, update profile
- Documented web-only limitations (role management requires tootctl)

Constitution Publishing:
- Publish constitution as pinned Mastodon thread
- Automatic deprecation of previous versions
- Version control with change summaries
- Thread splitting for long documents
- CLI tool: scripts/publish_constitution.py
- Documentation: CONSTITUTION_PUBLISHING.md

Configuration & Integration:
- Added SlackConfig model with bot_token, app_token, channel_id
- Updated PlatformConfig to support type: slack
- Added slack-sdk>=3.33.0 dependency
- Bot.py now routes to Slack adapter
- Updated example config with Slack section

Documentation:
- SLACK_SETUP.md: Complete Slack setup guide
- PLATFORM_SKILLS.md: All 35+ Mastodon skills documented
- CONSTITUTION_PUBLISHING.md: Constitution publishing guide
- Updated README.md: Merged QUICKSTART, added Slack to supported platforms
- Updated PLATFORMS.md: Slack marked as implemented with examples
- Updated .gitignore: Added instance-specific state files

Security:
- All sensitive files properly gitignored
- Instance-specific state (.constitution_post_id) excluded
- Credentials properly handled in config

Breaking Changes: None

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

config/config.example.yaml

@@ -1,5 +1,5 @@
 platform:
-  # Platform type: mastodon, discord, telegram, mock
+  # Platform type: mastodon, slack, discord, telegram, mock
   type: mastodon
 
   # Mastodon configuration (if using Mastodon)
@@ -10,6 +10,12 @@ platform:
     access_token: your_access_token_here
     bot_username: govbot
 
+  # Slack configuration (if using Slack)
+  # slack:
+  #   bot_token: xoxb-your-bot-oauth-token-here
+  #   app_token: xapp-your-app-level-token-here
+  #   channel_id: C0123456789
+
   # Discord configuration (for future use)
   # discord:
   #   token: your_discord_bot_token