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.

test/install-mcp-server.test.ts

@@ -32,6 +32,7 @@ describe("prepareMcpConfig", () => {
       useStickyComment: false,
       useCommitSigning: false,
       allowedBots: "",
+      trackProgress: false,
     },
   };