[CLEAN] Synthetic Benchmark PR #26636 - feat: implement git checkout caching to speed up CI workflows

[ofir-frd]

Benchmark PR calcom#26636

Type: Clean (correct implementation)

Original PR Title: feat: implement git checkout caching to speed up CI workflows
Original PR Description: ## What does this PR do?

Implements git checkout caching to reduce CI workflow times by eliminating redundant git checkouts across parallel jobs.

Previously, every job in the PR workflow performed its own full git checkout (via actions/checkout@v4 + dangerous-git-checkout), resulting in 20+ separate checkouts per workflow run. This was particularly slow for the cal.com repo (~1.3GB).

This PR:

• Creates a new cache-checkout action that saves/restores the git working directory
• The prepare job saves the checkout to cache after dangerous-git-checkout
• All downstream workflows use sparse-checkout of .github (to access the action) then restore from cache
• Uses cache key format git-checkout-{branch}-{sha} to ensure cache is only reused within a single PR
• Deletes previous caches for the branch before saving to keep cache storage low (only one cache per branch)
• Automatically cleans up git-checkout caches when PRs are closed (via delete-blacksmith-cache.yml)
• Uses sparse-checkout in dangerous-git-checkout to exclude /example-apps/ and *.mp4 files (~38MB saved from salesroom demo video)

Expected impact: Reduces checkout time from ~10+ minutes per job to a fast cache restore, with only one full checkout per workflow run.

Link to Devin run: https://app.devin.ai/sessions/00c0c2b934dc40609aaf765ab206daaa
Requested by: @keithwillcode

Updates since last revision

• Fixed cache key to use github.head_ref || github.ref_name and github.sha instead of github.event.pull_request.head.ref/sha (the latter is not available in workflow_call contexts)
• Final cache key format: git-checkout-${{ github.head_ref || github.ref_name }}-${{ github.sha }}
• Added prefix delete step before saving cache to clear previous checkout caches for the same branch (uses useblacksmith/cache-delete@v1 with prefix: "true")
• Added cache cleanup job in delete-blacksmith-cache.yml to delete all git-checkout caches with the PR branch prefix when PR is closed
• Added sparse-checkout to dangerous-git-checkout action to exclude example-apps/ directory and all .mp4 files
• Fixed: Restored get_sha step and commit-sha output that was accidentally removed when adding the cache-checkout step (this was breaking the ready-for-e2e label check)
• Fixed: Added trailing - to cache key prefix to prevent accidental deletion of other branches' caches (e.g., branch feature no longer deletes caches for feature-2)

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A - CI infrastructure change only.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

This PR modifies CI infrastructure and can only be tested by running the actual workflows:

1. Create/update a PR to trigger the workflow
2. Verify the prepare job successfully deletes old caches and saves the checkout to cache
3. Verify downstream jobs (lint, type-check, unit-tests, etc.) successfully restore from cache
4. Push another commit and verify the old cache is deleted before the new one is saved
5. Compare total workflow time before and after this change
6. Close a PR and verify the git-checkout caches are cleaned up

Human Review Checklist

• Verify fail-on-cache-miss: true is acceptable behavior (downstream jobs will fail if prepare job fails before caching)
• Confirm cache paths correctly exclude .git and node_modules
• Verify github.head_ref || github.ref_name and github.sha are available in all workflow_call contexts
• Consider edge cases: what happens if cache is evicted mid-workflow?
• Confirm sparse-checkout of .github is fast enough to be worthwhile vs. the previous approach
• Verify cache key handles branch names with special characters correctly
• Confirm prefix-based cache deletion (git-checkout-{branch}) won't accidentally delete unrelated caches - Fixed by adding trailing - to prefix
• Verify no workflows need files from example-apps/ or the salesroom mp4 file
• Confirm sparse-checkout patterns (/*, !/example-apps/, !**/*.mp4) work correctly in non-cone mode
• Verify get_sha step and commit-sha output are properly restored in pr.yml

Checklist

• My code follows the style guidelines of this project
• I have checked if my changes generate no new warnings
  Original PR URL: feat: implement git checkout caching to speed up CI workflows calcom/cal.com#26636

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Available types:
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