feat: enhance mode routing with track_progress and context preservation

This PR implements enhanced mode routing to address two critical v1 migration issues:
1. Lost GitHub context when using custom prompts in tag mode
2. Missing tracking comments for automatic PR reviews

Changes:
- Add track_progress input to force tag mode with tracking comments for PR/issue events
- Support custom prompt injection in tag mode via <custom_instructions> section
- Inject GitHub context as environment variables in agent mode
- Validate track_progress usage (only allowed for PR/issue events)
- Comprehensive test coverage for new routing logic

Event Routing:
- Comment events: Default to tag mode, switch to agent with explicit prompt
- PR/Issue events: Default to agent mode, switch to tag mode with track_progress
- Custom prompts can now be used in tag mode without losing context

This ensures backward compatibility while solving context preservation and tracking visibility issues reported in discussions #490 and #491.

src/github/context.ts

@@ -75,6 +75,7 @@ type BaseContext = {
     useStickyComment: boolean;
     useCommitSigning: boolean;
     allowedBots: string;
+    trackProgress: boolean;
   };
 };
 
@@ -122,6 +123,7 @@ export function parseGitHubContext(): GitHubContext {
       useStickyComment: process.env.USE_STICKY_COMMENT === "true",
       useCommitSigning: process.env.USE_COMMIT_SIGNING === "true",
       allowedBots: process.env.ALLOWED_BOTS ?? "",
+      trackProgress: process.env.TRACK_PROGRESS === "true",
     },
   };