fix(subagents): drain spawn hook stdout before exiting

[thisisjun786]

Problem

Large spawn-hook responses are truncated when stdout is a pipe on POSIX. The CLI calls process.exit(0) immediately after process.stdout.write(out), before the asynchronous write drains. This can discard the entire updatedInput rewrite, including configured subagent model/effort and attached skill instructions.

A real three-skill spawn produced 81,754 bytes, but the installed CLI emitted only 65,536 bytes and exited 0. Changing pipe capacity to 4 KiB or 8 KiB moved the truncation boundary accordingly.

Change

Allow pending stdout writes to finish by using process.exitCode = 0 and natural exit. Regenerate the shipped JavaScript and add real CLI regression coverage for large piped JSON, including model/effort and message preservation.

Validation

• Linux / Node 24.20.0: restored the original exit temporarily; the new CLI regression fails with truncated/unterminated JSON. Restored the patch; source and committed dist both pass.
• Focused spawn-hook suite: 80 passed, 0 failed.
• Full npm test: 2,580 total, 2,509 passed, 0 failed, 71 skipped. Used an isolated TMPDIR because this host has an unrelated /tmp/.git marker that affects the existing project-root tests.
• npm run build, inventory check with measured total 2,580, and repository gate: passed. README test badges were regenerated as required by CI.
• Patched installed CLI: complete 81,754-byte JSON at 4/8/64 KiB pipe capacity.
• Live fresh worker spawn with three skill attachments and no explicit model/effort: project-configured model/effort preserved; worker ran six checks, wrote its evidence receipt, and completed normally.

No role detection, configuration schema, or policy changes. Local live verification is Linux-only; the existing CI matrix covers macOS and Windows.