fix(interceptors): relay SIGTERM and SIGHUP to the wrapped command

[moise10r]

What this changes

Stopping a wrapper left the real command running. memnox-shell and memnox-intercept now forward SIGTERM and SIGHUP to the command and stop listening once it exits. memnox-intercept exits 143 or 129 rather than 1. The shared helpers live in a new child-signals.ts.

How it was verified

packages/interceptors/test/child-signals.test.ts sends SIGTERM and SIGHUP to a real sleep 30 through an injected source. It checks the exit code, that the process is gone and that the listeners are removed. The test exercises the shared helper rather than the built binaries.

Fixes #17

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)