feat: add modular sub-actions for parallel review workflow

[shashank-factory]

Summary

Add modular sub-actions (prepare/, review/, combine/) that enable parallel execution of code reviews in multi-job GitHub Actions workflows.

Changes

New Sub-Actions

• prepare/action.yml - Initialize review, create tracking comment, detect review modes
• review/action.yml - Run code review as standalone job
• combine/action.yml - Combine results and post inline comments

New Entrypoints

• src/entrypoints/get-token.ts - Get GitHub token via OIDC for sub-actions
• src/entrypoints/generate-review-prompt.ts - Generate review prompt for standalone review action
• src/entrypoints/generate-combine-prompt.ts - Generate combine prompt for result aggregation

Type Changes

• Made commentId optional in create-prompt types to support sub-action usage
• Added skipped and reason fields to PrepareResult for better flow control

Testing

• All existing tests pass
• Type checking passes

Usage

This enables workflows like:

jobs:
  prepare:
    steps:
      - uses: Factory-AI/droid-action/prepare@v1
        with:
          factory_api_key: ${{ secrets.FACTORY_API_KEY }}

  code-review:
    needs: prepare
    steps:
      - uses: Factory-AI/droid-action/review@v1
        with:
          factory_api_key: ${{ secrets.FACTORY_API_KEY }}
          tracking_comment_id: ${{ needs.prepare.outputs.comment_id }}

  combine:
    needs: [prepare, code-review]
    steps:
      - uses: Factory-AI/droid-action/combine@v1
        with:
          factory_api_key: ${{ secrets.FACTORY_API_KEY }}
          tracking_comment_id: ${{ needs.prepare.outputs.comment_id }}

Part of

This is PR 1 of 3 for the security review feature:

1. This PR: Sub-action infrastructure
2. PR 2: Code review refactor (output to JSON)
3. PR 3: Security review feature

[factory-droid]

Droid finished @shashank-factory's task —— View job

---

Code Review Completed

✅ No high-severity issues found

Summary

This PR introduces sub-action infrastructure for the Droid GitHub Action, splitting functionality into three composable actions (prepare/, review/, combine/) with dedicated TypeScript entrypoints.

Key Changes Reviewed

• New composite actions in prepare/action.yml, review/action.yml, and combine/action.yml
• New entrypoints: get-token.ts, generate-review-prompt.ts, generate-combine-prompt.ts
• Type changes making droidCommentId and commentId optional throughout the prompt creation system
• New combine-prompt.ts template for finalizing reviews

Files Changed

File	Additions	Deletions
combine/action.yml	71	0
prepare/action.yml	77	0
review/action.yml	83	0
src/entrypoints/generate-combine-prompt.ts	110	0
src/entrypoints/generate-review-prompt.ts	126	0
src/entrypoints/get-token.ts	40	0
src/create-prompt/templates/combine-prompt.ts	83	0
src/create-prompt/index.ts	10	15
src/create-prompt/types.ts	1	1
src/prepare/types.ts	2	0

Review Notes

• Code patterns are consistent with existing codebase conventions
• Type changes properly propagate the optional droidCommentId through the prompt creation system
• New entrypoints follow established error handling patterns

View workflow run