repl: support group-qualified .scenario in multi-API sessions

[Copilot]

Summary

Extended REPL .scenario to be mode-aware: single-runner keeps .scenario <path>, while multi-API now requires .scenario <group> <path> and executes against the selected runner’s scenario registry/context helpers.

• Command parsing
  • Added dual syntax handling by mode (<path> vs <group> <path>).
  • Preserved single-runner path semantics (entire argument remains the scenario path).
• Runner-qualified execution
  • Scenario module/function resolution now uses the selected group binding in multi-API mode.
  • Scenario function receives the selected binding’s context, loadContext, route, and routes.
• Error/usage UX
  • Unknown group errors now include available group keys.
  • Usage output is mode-specific.
• Docs/tests
  • Updated REPL docs with multi-API .scenario <group> <path> usage.
  • Added unit coverage for multi-API parsing, unknown group handling, usage errors, and group-isolated context/routes behavior.

// Multi-API REPL
.scenario billing soldPets

Original Prompt

Issue: repl: add group-qualified .scenario command for multi-API

Extend .scenario so multi-API sessions can choose which API runner's scenario registry and context are used.

Context

Scenario scripts are loaded per runner (<base>/<group>/scenarios). In multi-runner mode, .scenario needs an API qualifier to avoid ambiguity and to execute against the intended context/route helpers.

Proposed change

Add dual command syntax:

• single-runner: .scenario <path> (existing behavior)
• multi-runner: .scenario <group> <path>

The command should resolve the selected group, load that group's scenario module, and pass the group's context, loadContext, route, and routes into the scenario function.

Acceptance criteria

• .scenario <group> <path> executes scenarios for the selected group in multi-runner mode
• Unknown group names produce a clear error message with available group keys
• Invalid or missing arguments print mode-appropriate usage help
• Single-runner .scenario <path> behavior remains unchanged
• Unit tests cover parsing, error cases, and context binding for both single- and multi-runner modes

Manual acceptance tests

• In a single-API session, .scenario soldPets still applies the same scenario function as before.
• In a multi-API session, .scenario billing soldPets applies only billing context/routes and does not mutate other groups.
• In a multi-API session, .scenario payments soldPets shows an unknown-group error listing available groups.
• In a multi-API session, invalid args (e.g. .scenario, .scenario billing) show usage: .scenario <group> <path>.
• Path traversal input (e.g. .scenario billing ../seed) is rejected with the existing path-segment error.

Tasks

• Updated .scenario command handling in src/repl/repl.ts to support mode-aware argument parsing and group-qualified execution.
• Wired scenario invocation in multi-API mode to the selected binding’s scenario registry + context helpers (context, loadContext, route, routes).
• Added unknown-group and mode-specific usage messaging.
• Expanded .scenario REPL tests in test/repl/repl.test.ts for multi/single parsing, error handling, and group isolation.
• Updated docs/features/repl.md with multi-API .scenario <group> <path> usage.
• Added a patch changeset describing the REPL behavior update.