fix: preserve gbrain argv on Windows without a shell

[stslgn]

Summary

• run direct gbrain invocations through cross-spawn without shell: true
• centralize synchronous, text, and asynchronous argv handling
• preserve source IDs, paths, and search arguments as literal argv values
• exercise the regression in the existing windows-latest test job

Why

On Windows, launching the gbrain.cmd shim through Node's shell mode can reinterpret argument metacharacters. cross-spawn resolves the command shim while keeping the argument vector separate from a composed shell command.

This is a focused alternative to #2272. That PR includes broader Windows runtime and release changes; this one intentionally limits scope to direct gbrain process creation and its regression coverage.

Validation

• bun run build
• targeted gbrain tests: 38 passed, 0 failed
• full bun run test completed locally with exit 0; relevant targeted tests are clean

Windows CI now runs both the argv round-trip and shell-free adapter invariants.

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[stslgn]

CI is waiting for first-time-contributor approval: all seven pull-request workflows currently report action_required, including Windows Free Tests, which is the key gate for this change.

Could a maintainer please approve and run the workflows? I will keep the PR in Draft until the Windows job is green.

[garrytan]

Closing: this was absorbed or superseded on main by v1.67.0.0 or earlier (see the v1.67 tracker-wave receipts in PR #2604). Thank you!