feat(runner): classify killed tests (timeout vs OOM vs SIGKILL)

[Chemaclass]

Goal

Distinguish causes behind the generic "Killed" message.

Why

src/runner.sh L132-150 emits a generic "Killed" — ambiguous between timeout, OOM, and SIGKILL.

Notes

• Map exit/signal codes to clearer messages.
• TDD: RED → GREEN → REFACTOR. Bash 3.0+ compatible. Update CHANGELOG.

Part of #677.

[Chemaclass]

Done in #689 (merged).