fix(task): optionally consume prompt files

[fscfede-beep]

Summary

Fixes #622 by adding an opt-in --prompt-file-consume flag to task.

When the flag is used with --prompt-file, the companion now reads the prompt, unlinks the caller-provided file, and only then proceeds to task dispatch. Without the flag, existing --prompt-file behavior is unchanged.

Semantics

The ordering is deliberately:

READ -> DELETE -> DISPATCH

• if the read fails, no deletion is attempted;
• if the unlink fails, task dispatch does not proceed;
• without --prompt-file-consume, the prompt file remains caller-owned as before;
• --prompt-file-consume without --prompt-file fails with a clear error.

This implements the smallest opt-in fix proposed in #622 and does not mix in prompt digest verification or SessionEnd lifecycle changes.

Regression coverage

Added three runtime tests:

1. consume mode deletes the prompt file after a successful read and the exact prompt still reaches turn/start;
2. ordinary --prompt-file preserves the file;
3. consume mode without --prompt-file is rejected.

RED against upstream/main (db52e28f4d9ded852ab3942cea316258ae4ef346): 1 pass / 2 fail, with the file remaining and the invalid flag combination exiting 0.

GREEN with this patch: 3/3 pass.

Validation

• focused prompt-file runtime tests: 3/3 pass
• non-runtime suites (bump-version, commands, git, process, render, state): 28/28 pass
• node --check plugins/codex/scripts/codex-companion.mjs: pass
• npx tsc -p tsconfig.app-server.json: pass
• npm run check-version: pass
• git diff --check: pass
• high-confidence secret/private-marker scan of the diff: pass

I also attempted the full Windows suite. It does not terminate cleanly because the current test harness leaves app-server broker/fake-Codex processes behind; the first visible failure, createBrokerEndpoint uses Unix sockets on non-Windows platforms, reproduces identically on an untouched upstream/main worktree. I therefore am not claiming a full-suite pass from this Windows host.

Scope

Two files only:

• plugins/codex/scripts/codex-companion.mjs
• tests/runtime.test.mjs

No version bump or unrelated lifecycle changes.

[sylvesterkaczmarek]

READ -> DELETE -> DISPATCH is the correct ownership order for an opt-in consume mode: a failed read preserves the caller's file, a failed unlink prevents execution, and successful dispatch cannot leave the supposedly consumed prompt behind. Keeping ordinary --prompt-file unchanged avoids surprising existing callers. The three regressions pin all of those branches cleanly.