feat: add configurable concurrency and improve file copying

[ghiscoding]

Summary

Limit simultaneous file copies, close streams reliably after failures, reduce repeated filesystem work, and honor string exclusions.

Why

Large batches could exhaust file descriptors, and failed writes leaked open streams. Discovery repeated unnecessary filesystem checks, while string exclusions were silently ignored.

Changes

• Default concurrency to os.availableParallelism(), capped at 32.
• Add validated API concurrency and CLI --concurrency / -c overrides.
• Stop queued copies on failure and close active streams before reporting the original error.
• Cache repeated glob results and destination-directory checks; use Dirents to avoid per-file stats.
• Normalize string exclusions while preserving default exclusions and ordered negation.
• Add regression tests and document concurrency and failure behavior.

Validation

• TypeScript build and Biome checks passed.
• Vitest: 85 passed, 1 skipped; 100% line coverage.
• Verified both CLI concurrency flags and rejection of invalid values.
• Confirmed copying succeeds under a 256-descriptor limit and repeated failed writes no longer leak descriptors.
• git diff --check passed.

Comments

Discovery remains synchronous. Files already written may remain after a copy fails.

AI / LLM assistance

• AI / LLM assistance used:
  • No
  • Yes
• If Yes:
  • which tool/model: OpenAI ChatGPT 6 Astra
  • how was it used: Auditing, benchmarking, implementation, regression tests, and documentation.

Checklist

• The changes are limited to only one scope.
• Tests were added or updated where appropriate.
• Documentation was updated where appropriate.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (164d340) to head (2fbbf10).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #97   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          206       224   +18     
  Branches        68        70    +2     
=========================================
+ Hits           206       224   +18     

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