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>

PLATFORMS.md

@@ -4,7 +4,16 @@ This guide explains how to implement Govbot adapters for new social/communicatio
 
 ## Overview
 
-Govbot uses a **platform-agnostic architecture** that separates governance logic from platform-specific code. This allows the same constitutional reasoning and governance processes to work across different platforms (Mastodon, Discord, Telegram, Matrix, etc.).
+Govbot uses a **platform-agnostic architecture** that separates governance logic from platform-specific code. This allows the same constitutional reasoning and governance processes to work across different platforms.
+
+**Currently Implemented**:
+- ✅ **Mastodon** - Instance-wide governance with full admin/moderation API
+- ✅ **Slack** - Channel-scoped governance with Socket Mode
+
+**Planned**:
+- 🚧 **Discord** - Server-wide governance (see example skeleton below)
+- 🚧 **Telegram** - Group governance
+- 🚧 **Matrix** - Room governance
 
 The key abstraction is the **PlatformAdapter** interface, which defines how Govbot interacts with any platform.
 
@@ -841,8 +850,10 @@ class DiscordAdapter(PlatformAdapter):
 
 ## Getting Help
 
-- Review the Mastodon adapter for a complete example
-- Check the base PlatformAdapter for interface documentation
+- Review the **Mastodon adapter** (`src/govbot/platforms/mastodon.py`) for an instance-wide governance example
+- Review the **Slack adapter** (`src/govbot/platforms/slack.py`) for a channel-scoped governance example
+- Check the base PlatformAdapter (`src/govbot/platforms/base.py`) for interface documentation
+- See setup guides: [MASTODON_SETUP.md](MASTODON_SETUP.md), [SLACK_SETUP.md](SLACK_SETUP.md)
 - Look at test files for testing patterns
 - Ask questions in project discussions