feat(#170): Implement mosaic-bridge module for Discord

Created the mosaic-bridge module to enable Discord integration for
chat-based control of Mosaic Stack. This module provides the foundation
for receiving commands via Discord and forwarding them to the stitcher
for job orchestration.

Key Features:
- Discord bot connection and authentication
- Command parsing (@mosaic fix, status, cancel, verbose, quiet, help)
- Thread management for job updates
- Chat provider interface for future platform extensibility
- Noise management (low/medium/high verbosity levels)

Implementation Details:
- Created IChatProvider interface for platform abstraction
- Implemented DiscordService with Discord.js
- Basic command parsing (detailed parsing in #171)
- Thread creation for job-specific updates
- Configuration via environment variables

Commands Supported:
- @mosaic fix <issue> - Start job for issue
- @mosaic status <job> - Get job status (placeholder)
- @mosaic cancel <job> - Cancel running job (placeholder)
- @mosaic verbose <job> - Stream full logs (placeholder)
- @mosaic quiet - Reduce notifications (placeholder)
- @mosaic help - Show available commands

Testing:
- 23/23 tests passing (TDD approach)
- Unit tests for Discord service
- Module integration tests
- 100% coverage of critical paths

Quality Gates:
- Typecheck: PASSED
- Lint: PASSED
- Build: PASSED
- Tests: PASSED (23/23)

Environment Variables:
- DISCORD_BOT_TOKEN - Bot authentication token
- DISCORD_GUILD_ID - Server/Guild ID (optional)
- DISCORD_CONTROL_CHANNEL_ID - Channel for commands

Files Created:
- apps/api/src/bridge/bridge.module.ts
- apps/api/src/bridge/discord/discord.service.ts
- apps/api/src/bridge/interfaces/chat-provider.interface.ts
- apps/api/src/bridge/index.ts
- Full test coverage

Dependencies Added:
- discord.js@latest

Next Steps:
- Issue #171: Implement detailed command parsing
- Issue #172: Add Herald integration for job updates
- Future: Add Slack, Matrix support via IChatProvider

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

.env.example

@@ -163,6 +163,15 @@ GITEA_REPO_NAME=stack
 # Configure in Gitea: Repository Settings → Webhooks → Add Webhook
 GITEA_WEBHOOK_SECRET=REPLACE_WITH_RANDOM_WEBHOOK_SECRET
 
+# ======================
+# Discord Bridge (Optional)
+# ======================
+# Discord bot integration for chat-based control
+# Get bot token from: https://discord.com/developers/applications
+# DISCORD_BOT_TOKEN=your-discord-bot-token-here
+# DISCORD_GUILD_ID=your-discord-server-id
+# DISCORD_CONTROL_CHANNEL_ID=channel-id-for-commands
+
 # ======================
 # Logging & Debugging
 # ======================