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.

action.yml

@@ -73,6 +73,10 @@ inputs:
     description: "Enable commit signing using GitHub's commit signature verification. When false, Claude uses standard git commands"
     required: false
     default: "false"
+  track_progress:
+    description: "Force tag mode with tracking comments for pull_request and issue events. Only applicable to pull_request (opened, synchronize, ready_for_review, reopened) and issue (opened, edited, labeled, assigned) events."
+    required: false
+    default: "false"
   experimental_allowed_domains:
     description: "Restrict network access to these domains only (newline-separated). If not set, no restrictions are applied. Provider domains are auto-detected."
     required: false
@@ -140,6 +144,7 @@ runs:
         USE_STICKY_COMMENT: ${{ inputs.use_sticky_comment }}
         DEFAULT_WORKFLOW_TOKEN: ${{ github.token }}
         USE_COMMIT_SIGNING: ${{ inputs.use_commit_signing }}
+        TRACK_PROGRESS: ${{ inputs.track_progress }}
         ADDITIONAL_PERMISSIONS: ${{ inputs.additional_permissions }}
         CLAUDE_ARGS: ${{ inputs.claude_args }}
         ALL_INPUTS: ${{ toJson(inputs) }}
@@ -247,6 +252,7 @@ runs:
         PREPARE_ERROR: ${{ steps.prepare.outputs.prepare_error || '' }}
         USE_STICKY_COMMENT: ${{ inputs.use_sticky_comment }}
         USE_COMMIT_SIGNING: ${{ inputs.use_commit_signing }}
+        TRACK_PROGRESS: ${{ inputs.track_progress }}
 
     - name: Display Claude Code Report
       if: steps.prepare.outputs.contains_trigger == 'true' && steps.claude-code.outputs.execution_file != ''