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>

apps/api/src/bridge/interfaces/chat-provider.interface.ts

@@ -0,0 +1,79 @@
+/**
+ * Chat Provider Interface
+ *
+ * Defines the contract for chat platform integrations (Discord, Slack, Matrix, etc.)
+ */
+
+export interface ChatMessage {
+  id: string;
+  channelId: string;
+  authorId: string;
+  authorName: string;
+  content: string;
+  timestamp: Date;
+  threadId?: string;
+}
+
+export interface ChatCommand {
+  command: string;
+  args: string[];
+  message: ChatMessage;
+}
+
+export interface ThreadCreateOptions {
+  channelId: string;
+  name: string;
+  message: string;
+}
+
+export interface ThreadMessageOptions {
+  threadId: string;
+  content: string;
+}
+
+export interface VerbosityLevel {
+  level: "low" | "medium" | "high";
+  description: string;
+}
+
+/**
+ * Chat Provider Interface
+ *
+ * All chat platform integrations must implement this interface
+ */
+export interface IChatProvider {
+  /**
+   * Connect to the chat platform
+   */
+  connect(): Promise<void>;
+
+  /**
+   * Disconnect from the chat platform
+   */
+  disconnect(): Promise<void>;
+
+  /**
+   * Check if the provider is connected
+   */
+  isConnected(): boolean;
+
+  /**
+   * Send a message to a channel or thread
+   */
+  sendMessage(channelId: string, content: string): Promise<void>;
+
+  /**
+   * Create a thread for job updates
+   */
+  createThread(options: ThreadCreateOptions): Promise<string>;
+
+  /**
+   * Send a message to a thread
+   */
+  sendThreadMessage(options: ThreadMessageOptions): Promise<void>;
+
+  /**
+   * Parse a command from a message
+   */
+  parseCommand(message: ChatMessage): ChatCommand | null;
+}

apps/api/src/bridge/interfaces/index.ts

@@ -0,0 +1 @@
+export * from "./chat-provider.interface";