test(release): cover default runner path

[steipete]

Summary

• exercise the release packager happy path with MCP_RUNNER explicitly unset
• retain the override-based failure cases for fast, hermetic setup
• make a stale or missing default runner fail the release contract suite

Proof

• ./scripts/test-release.sh
• mutation check: changed the default runner to a nonexistent path and confirmed the release contract test failed with exit 127
• pnpm check
• pnpm test
• autoreview clean with Codex gpt-5.6-sol

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: test(release): cover default runner path This is item 1/1 in the current shard. Shard 0/1.

This placeholder means the worker is alive and reading the current context. I will edit this same comment with the actual review when the claws are done clicking.

Crustacean status: shell secured, claws on keyboard, evidence pebbles being sorted.