feat(vectorise): add --batch-size option for memory-efficient processing

[superbiche]

Summary

Adds a --batch-size / -b option to the vectorise command that controls how many files are processed concurrently.

• Default: 100 files per batch
• Use -1 to disable batching (process all files at once, legacy behavior)

This addresses memory exhaustion issues when vectorizing large codebases with thousands of files, as it limits the number of concurrent async tasks created.

Changes

• Added batch_size field to Config dataclass with default value of 100
• Added -b/--batch_size CLI argument to vectorise subparser
• Modified vectorise() to process files in batches using the configured batch size
• Added tests for batch processing functionality

Test plan

• pytest tests/subcommands/test_vectorise.py -v - all tests pass
• Manual test: vectorcode vectorise /large/project -r -b 50 processes files in batches
• Manual test: vectorcode vectorise /project -r -b -1 processes all files at once (legacy)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.72%. Comparing base (3eacce5) to head (4d289b0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #306   +/-   ##
=======================================
  Coverage   99.72%   99.72%           
=======================================
  Files          25       25           
  Lines        1845     1851    +6     
=======================================
+ Hits         1840     1846    +6     
  Misses          5        5           

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