ci: share rust cache across branches and restore file mtimes

[desmondcheongzx]

Two changes to fix integration-test-build timeouts caused by Rust cache eviction and cargo fingerprint invalidation:

Shared cache key with main-only saves. Switches Swatinem/rust-cache from prefix-key (branch-scoped) to shared-key + save-if: main. All PR branches read from the same cache entry that only main writes, eliminating per-branch duplication that was filling the 10GB GitHub Actions cache limit and causing evictions.

Restore file mtimes after checkout. actions/checkout sets all file mtimes to the checkout timestamp, which invalidates cargo fingerprints even when Swatinem restores a valid target/ directory — causing cargo to recompile all ~70 local crates from scratch. A lightweight git log --raw script restores each file's mtime to its last-commit timestamp, making fingerprints match across CI runs.

The first main build after merge will be a cold build that saves the cache with the new shared key format. Subsequent builds (main and PR) should only recompile changed crates.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.43%. Comparing base (ee16fc9) to head (0242fb0).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6246      +/-   ##
==========================================
+ Coverage   73.37%   73.43%   +0.05%     
==========================================
  Files         999     1001       +2     
  Lines      132632   133095     +463     
==========================================
+ Hits        97318    97732     +414     
- Misses      35314    35363      +49     

see 49 files with indirect coverage changes

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[greptile-apps]

Greptile Summary

This PR implements two complementary optimizations to reduce integration-test-build CI time from ~39 minutes to ~10-15 minutes:

• Shared cache across branches: Switches from prefix-key to shared-key + save-if to prevent cache duplication across branches. PRs will read from the shared cache without writing, saving ~3.3 GB of the 10 GB cache limit.
• File mtime restoration: Adds a post-checkout step that restores each file's mtime to its last commit timestamp using git log. This prevents cargo from unnecessarily recompiling all ~70 local crates when the cache fingerprints would otherwise match.

Both changes are scoped to the integration-test-build job as an experiment before potential wider rollout. The implementation is clean and follows CI optimization best practices.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• Score reflects well-tested CI optimization patterns (mtime restoration is a known cargo caching technique, shared cache keys are standard). Changes are scoped to a single job as an experiment, limiting blast radius. The shell script is deterministic and handles edge cases properly (checks file existence before touching). No functional code changes.
• No files require special attention

Important Files Changed

Filename	Overview
.github/workflows/pr-test-suite.yml	Adds mtime restoration after checkout and switches integration-build cache from prefix-key to shared-key with save-if restriction to main branch only

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Checkout Code] --> B[Restore File Mtimes]
    B --> C{Git Log Parser}
    C --> D[AWK: Map Files to Commit Timestamps]
    D --> E[Touch Files with Timestamps]
    E --> F[Setup Rust Cache]
    F --> G{Is Main Branch?}
    G -->|Yes| H[Read/Write Shared Cache]
    G -->|No| I[Read-Only Shared Cache]
    H --> J[Cargo Build with Valid Fingerprints]
    I --> J
    J --> K[Fast Build - No Recompilation]
    
    style B fill:#90EE90
    style G fill:#FFD700
    style K fill:#90EE90

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