feat(ci): add selective backend/web/mobile checks with PR comments

[Harxhit]

Summary

Adds a selective monorepo CI workflow that detects changed files and runs domain-specific checks for backend, web, and mobile applications. This improves CI efficiency by avoiding unnecessary checks and introduces automated PR result comments summarizing CI outcomes. Mobile emulator/device build validation is intentionally deferred for future work.

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Type of Change

• Bug fix
• New feature
• Refactor (no functional change)
• UI / Design change
• Tests only
• Documentation
• Infrastructure / DevOps
• Security

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What Changed

• Added change-detection logic using GitHub API (pulls.listFiles) to identify modified backend, web, and mobile files.
• Added selective CI jobs for backend, web, and mobile that run only when corresponding files change.
• Added PR comment automation to summarize CI results across backend, web, and mobile jobs.

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How to Test

1. Open or update a PR with changes in apps/backend, apps/web, or apps/mobile.
2. Verify only the relevant CI jobs run based on modified files.
3. Confirm the workflow posts a PR comment summarizing CI job results.

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Checklist

• My code follows the project's coding style (pnpm -r run lint passes).
• TypeScript compiles without errors (pnpm -r run typecheck).
• I have added or updated tests for the changes I made.
• All tests pass locally (pnpm -r run test).
• I have updated documentation where necessary.
• No new console.log or debug statements left in the code.
• Breaking changes are documented in this PR description.

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Screenshots / Recordings

N/A

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Additional Context

• CI currently validates backend, web, and mobile lint/build/test workflows selectively.
• Mobile emulator/device execution and E2E validation are intentionally not included in this PR and are planned for future improvements.
• This setup is designed to scale with the monorepo and reduce unnecessary CI execution time.

[Harxhit]

@ShantKhatri Please check this.