test(repl): raise mutation score to 80%

[pmcelhaney]

Summary

• Add REPL behavior tests for root proxy messaging and deterministic proxy-status ordering.
• Cover dot-path rejection, scenario failures, and unavailable multi-API route bindings.
• Raise the REPL incremental mutation score from 78.05% to 80.08% (418/522 detected mutants).

Validation

• yarn workspace @counterfact/repl test
• yarn test:mutation:incremental --package repl
• yarn lint
• yarn typecheck

Manual acceptance tests

• With no proxy URL configured, enter .proxy on and confirm the REPL says root requests will use <proxy URL>/.
• Configure /zebra and /alpha proxy paths in reverse order, run .proxy status, and confirm the displayed paths are alphabetically ordered.
• Run .scenario ./setup and confirm the REPL rejects the dot path without applying a scenario.
• Run a scenario function that throws and confirm the REPL displays the error while remaining usable for the next command.
• In a multi-API REPL, invoke a scenario after its selected group has no routes binding and confirm the group-specific error is displayed.

Repository learning check

• Learning found: No
• Guidance updated: No
• Updated file(s): N/A
• Rationale: The change follows existing REPL harness and focused behavioral-test conventions without revealing a reusable repository rule that is not already documented.