fix(interceptors): report a signalled or unstartable child as a shell…

[moise10r]

What this changes

A child killed by a signal was reported as refused (77) or failed (1). It now exits 128 plus the signal number, in both memnox-shell and the interceptor. A missing binary exits 127 and a non-executable one exits 126, with the reason on stderr. Core gains exitCodeForChild and exitCodeForSpawnError and loses SIGNAL_NUMBER. This overlaps #17, so rebase whichever merges second.

How it was verified

packages/core/test/exit-code.test.ts spawns real children:

• One killed with SIGKILL gives 137.
• A missing binary gives 127.
• A file without the execute bit, in a temp dir, gives 126.

Fixes #16

Checklist

• pnpm format && pnpm typecheck && pnpm test && pnpm deadcode all pass
• Behaviour change ships with a test
• No any, no magic values, no console.* outside cli-output.ts
• If this touches the decision path: still deterministic — no LLM, network, or randomness
• If this changes a verb table: the classes that moved are named above (not applicable)
• If this changes a command, flag or file it writes: docs/ says so (not applicable)