fix(cli): fail unknown list targets

[steipete]

Summary

• Return exit code 1 for explicit mcporter list <unknown-server> failures.
• Keep aggregate list health output unchanged by default; unhealthy aggregate lists still only fail with --exit-code or --quiet.
• Add regression coverage for the unknown-server path and the aggregate default.

Closes #203.

Proof

• ./runner pnpm exec vitest run tests/cli-list-classification.test.ts tests/cli-list-json.test.ts
• ./runner pnpm build
• CLI proof: unknown explicit list target exits 1; aggregate list exits 0; aggregate --exit-code exits 1
• ./runner pnpm check
• ./runner pnpm test
• Autoreview: clean

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: fix(cli): fail unknown list targets 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.

[steipete]

Landed as 14ff39a.

Proof:

• ./runner pnpm exec vitest run tests/cli-list-classification.test.ts tests/cli-list-json.test.ts
• ./runner pnpm build
• manual CLI E2E: explicit unknown target exited 1; aggregate list --json exited 0; aggregate list --json --exit-code exited 1
• ./runner pnpm check
• ./runner pnpm test
• autoreview clean
• CI green: macOS, Ubuntu, Windows

Caveats: none.