Handle Deno subprocess stdin failures

[morluto]

Fixes #1415

Summary

The Deno subprocess runtime now observes and awaits stdin writes. If the child closes stdin during the initial protocol message or a tool-result write, execution returns a failed ExecuteResult instead of emitting an uncaught stream error or stalling.

Changes

• Own the child stdin error listener for the subprocess lifetime.
• Await write callbacks for the initial and tool-result protocol messages.
• Wait for the child spawn before sending the initial message.
• Dispose the child after a failed write.
• Add a subprocess fixture that closes stdin and a public-executor regression.
• Add an executor patch changeset.

Existing missing-Deno diagnostics remain unchanged.

Regression proof

On origin/main, the regression emits an uncaught EPIPE when the child closes stdin. When the uncaught event is intercepted by the harness, execution reaches its five-second guard without returning a result.

With this change, the same execution returns result: null with an actionable message containing Failed to write to Deno subprocess stdin: write EPIPE.

Validation

• Focused runtime suite: 3 tests passed.
• Fourteen real-Deno tests were skipped because Deno is not installed in the validation environment.
• Package typecheck passed with existing Effect language-service suggestions.
• Targeted lint, formatting, and git diff --check passed.
• Branch autoreview against origin/main returned no actionable findings.

Limitations

The regression uses a Node subprocess fixture to deterministically exercise the shared stdin failure path without requiring a Deno installation. The existing real-Deno suite remains unchanged and was skipped by the environment.

The fixture is skipped on Windows because its subprocess signaling is POSIX-specific.

Review order

1. The close-stdin fixture and public-executor regression.
2. Stdin ownership and write completion in deno-worker-process.ts.
3. Runtime wiring and release note.