fix: avoid shell:true deprecation on Windows

[fscfede-beep]

Summary

Fixes #717.

Related: #708, #525.

• stop defaulting shared process launches to shell: true on Windows
• route node / npm / codex shim-style probes through an explicit ComSpec /d /s /c call ... invocation with shell: false
• use the same Windows invocation for direct codex app-server startup
• invoke taskkill directly with shell: false, avoiding Git Bash/MSYS conversion of /PID, /T, and /F
• treat taskkill exit 128 as an already-stopped process, so cleanup does not depend on English error text
• add regression coverage for the Windows shim invocation, taskkill options, and localized missing-process output

This avoids Node's DEP0190 warning without breaking .cmd shims such as npm on Windows. It also addresses the shell/argument-conversion root cause reported in #708 and #525 and makes the already-dead taskkill path locale-independent.

Validation

On Windows 11 / Node 26.3.1:

• reproduced the pre-fix warning with node --trace-deprecation plugins/codex/scripts/codex-companion.mjs setup --json
• after the change, the same command exits 0 with stderr_bytes=0
• reproduced Git Bash/MSYS mangling on a nonexistent PID: taskkill /PID 999999 /T /F reaches taskkill as C:/Program Files/Git/PID and exits 1
• verified direct, no-shell taskkill /PID 999999 /T /F returns localized Spanish ERROR: no se encontró el proceso "999999". with exit 128
• node --test tests/process.test.mjs tests/commands.test.mjs tests/bump-version.test.mjs -> 14 passed, 0 failed
• node --check plugins/codex/scripts/lib/process.mjs
• node --check plugins/codex/scripts/lib/app-server.mjs
• git diff --check

I also verified on the same host that direct spawnSync(..., { shell: false }) works for .exe binaries but returns EINVAL for an absolute npm.cmd, which is why the patch keeps explicit cmd.exe handling for Windows command shims instead of globally assuming every command is directly executable.

[sylvesterkaczmarek]

Explicit ComSpec handling is the right Windows boundary: .cmd shims still need command-processor semantics, but the child-process API no longer relies on shell:true. Keeping taskkill direct also avoids MSYS argument rewriting, and exit 128 removes the locale dependency.