[Bug]: Custom Antigravity binary fails with wrapper scripts (.cmd/.bat) due to rigid localharness_external sibling check

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/desktop

Steps to reproduce

1. Create a custom wrapper script on Windows (e.g., agy-wrapper.cmd or a proxy executable) to wrap agy_acp_server.exe (e.g., to set custom environment variables or isolate temp directories).
2. Point Antigravity's custom binary path (binaryPath) to this wrapper.
3. Observe health check failure:
   The custom Antigravity executable or its localharness_external sibling is missing or not executable.
4. If a placeholder localharness_external.cmd or copy is placed alongside it, the health check fails or hangs with:
   Unavailable · Antigravity could not complete its local health check.

Expected behavior

1. T3 Code should allow custom binaries/wrappers without strictly requiring localharness_external.exe to reside in the exact same directory (e.g., fall back to the managed tool directory for localharness_external).
2. Windows process spawning for ACP should reliably handle standard I/O pipes (stdin/stdout) when invoking .cmd or proxy executables.

Actual behavior

resolveSiblingExternalBinary checks path.join(path.dirname(binaryPath), "localharness_external" + ext). If missing, it immediately rejects the binary. If present, standard I/O pipe inheritance through Windows cmd.exe /c breaks the ACP JSON-RPC handshake.

Impact

Minor bug or occasional failure

Version or commit

0.0.42 (and main)

Environment

Windows 11 x64, T3 Code Desktop 0.0.42

Logs or stack traces

Not found: The custom Antigravity executable or its localharness_external sibling is missing or not executable.
# Or:
Unavailable: Antigravity could not complete its local health check.

Workaround

Must invoke the raw unpacked agy_acp_server.exe directly from its directory with localharness_external.exe present alongside it; cannot use wrappers or intermediate launchers.

Suggested Fix

1. Make localharness_external resolution check the bundled/managed tool directory as a fallback when binaryPath is a custom path without its own localharness_external.
2. Ensure stdio options and process spawning on Windows correctly pipe stdio when binaryPath has a .cmd or .bat extension.

Start at resolveSiblingExternalBinary and the Windows ACP process-spawning path; trace the custom binary health check with a .cmd/.bat wrapper and the managed tool directory fallback. Done means custom wrappers can locate localharness_external through the fallback and ACP JSON-RPC completes reliably through standard I/O pipes.