feat: add opt-in checkout-bundle option to run-task

[hneiva]

Add a per-project --{project}-checkout-bundle=<dest> option to the run-task script. When set, run-task writes a .tar.zst archive of the checkout's working tree (excluding .git/.hg) right after checkout, so a decision task can publish its checkout as an artifact that downstream tasks fetch via fetches: instead of re-cloning.

The archive uses the same .tar.zst format as fetch-content's git-checkout-archive and is created fail-fast: archiving errors exit non-zero rather than leaving a broken artifact behind.

[codspeed-hq]

Merging this PR will improve performance by 12.27%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 23 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_transform_sequence	6.9 ms	6.1 ms	+12.27%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing hneiva/checkout-bundle (f1e3827) with main (81f3b38)}

[codecov]

Codecov Report

❌ Patch coverage is 94.56522% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.61%. Comparing base (81f3b38) to head (f1e3827).

Files with missing lines	Patch %	Lines
src/taskgraph/run-task/run-task	84.84%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #983      +/-   ##
==========================================
+ Coverage   78.53%   78.61%   +0.07%     
==========================================
  Files         132      132              
  Lines       12437    12460      +23     
  Branches     1510     1497      -13     
==========================================
+ Hits         9767     9795      +28     
+ Misses       2282     2269      -13     
- Partials      388      396       +8     

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