fix(scan): allow large git output during scan prep

[sylvesterkaczmarek]

Summary

Allow committed-diff scan preparation to handle repositories whose tracked-file inventory exceeds Node's default execFile stdout buffer.

validateCommittedDiffCheckout() runs git ls-files -t -z to detect sparse checkouts. On large repositories, that output can exceed Node's default 1 MiB buffer and terminate scan preparation with ERR_CHILD_PROCESS_STDIO_MAXBUFFER before scanning begins.

Fixes #565.

Changes

• remove Node's implicit 1 MiB stdout ceiling from the shared trusted Git execution path with maxBuffer: Infinity, matching the unbounded subprocess-buffer convention already used elsewhere in this package
• add an end-to-end regression that creates a synthetic clean Git repository whose tracked-file listing is confirmed to exceed 1 MiB
• exercise normal committed-diff target normalization and checkout validation against that repository
• keep Git invocation, sparse-checkout detection, target validation, and parsing semantics unchanged

Testing

• added a focused synthetic-repository regression that produces more than 1 MiB of real git ls-files -t -z output and verifies committed-diff validation completes
• reviewed the branch against current upstream main; the diff is limited to one production line in src/targets.ts and the regression file
• fork workflow execution did not produce a completed verification result in this environment, so pushed-head CI remains the authoritative full typecheck, formatting, build, and test validation

Risk and rollout

Low. Git command output was already collected in memory; this removes Node's default rejection threshold rather than changing the command, output format, or validation behavior. The change follows the same maxBuffer: Infinity convention already present in the SDK runtime and avoids introducing an arbitrary repository-size limit.

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