fix(acp): launch Windows command shims safely

[64ix]

Description

Route Windows .cmd and .bat ACP processes through cmd.exe with explicit argv escaping. This fixes npm-installed agents such as OpenCode failing with spawn EINVAL while preserving direct spawning for native executables and non-Windows hosts.

The same normalization is applied to ACP agent and terminal processes. shell=true is deliberately avoided.

Testing

• Runtime package build: passed
• Runtime typecheck: passed
• Targeted Windows regression tests: 3 passed
• Targeted oxlint: passed
• Targeted oxfmt check: passed
• Full runtime suite: 102 passed; 5 existing Windows-incompatible tests fail because they invoke POSIX sh/printf or assert /opt paths

Risk

ACP process spawning is security-sensitive. The wrapper is restricted to .cmd/.bat on Windows and explicitly escapes cmd.exe metacharacters.

Screenshot/Recording

Not applicable (process-launch fix).

Checklist

• I kept this PR small and focused
• I ran a self-review before opening this PR
• I ran the relevant local checks or explained why not
• I added regression tests
• I only added comments where the logic is not obvious
• I used a Conventional Commit title