refactor(github): split action.ts into smaller modules

[github-actions]

Closes #461

Applied changes based on Issue #461.

I’ve split the giant action.ts into four focused modules plus an aggregator, preserving all existing behavior and test contracts:

1. Progress UI (src/github/progress.ts)

• Moved escapeRegExp, createProgressComment and updateProgressComment into a dedicated progress‐comment manager.
• Centralizes all “spinner + checkbox” UI bits and related constants.

2. Prompt Builder (src/github/prompt-builder.ts)

• Extracted the logic around --fetch, --fix-build, createIssues, image‐URL extraction, and the generatePrompt call.
• Includes the helper fetchLatestFailedWorkflowLogs.

3. Result Handler (src/github/result-handler.ts)

• Pulled out the post‐Codex diff/PR/comment logic (handleResult), including workflow‐file filtering, image‐artifact cleanup, commit‐message generation, PR vs. comment dispatch, and --no-pr handling.

4. Run Action Orchestrator (src/github/runAction.ts)

• The former 900‑line runAction now only orchestrates the high‐level flow: reactions, clone, file‐snapshot, prompt preparation, Codex call, progress updates, create‑issues dispatch, change detection & progress ticks, then hands off to handleResult.

5. Action Aggregator (src/github/action.ts)

• Reduced to a simple index that re‑exports all of the above so imports remain unchanged and existing tests still pass.

export { escapeRegExp, createProgressComment, updateProgressComment } from './progress';
export { preparePrompt } from './prompt-builder';
export { handleResult } from './result-handler';
export { runAction } from './runAction';
export { createIssuesFromFeaturePlan } from './createIssues.js';

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Next steps:

• All existing imports (including in src/main.ts) continue to work without modification.
• The unit tests for escapeRegExp, createProgressComment, updateProgressComment, and createIssuesFromFeaturePlan will still pass against src/github/action.ts as before.
• No changes were made to tests or unrelated code.

Please let me know if you’d like any tweaks!