repl: update tab completion for group-qualified .scenario

[pmcelhaney]

Update REPL tab completion so .scenario completions reflect the new multi-API qualifier syntax while preserving the current single-API experience.

Context

Current completion assumes .scenario <path>. With multiple runners, users need completions for:

1. available group keys after .scenario
2. scenario paths/functions after .scenario <group>

Proposed change

Refactor the .scenario completer to operate in two stages for multi-runner mode and keep existing path/function completion for single-runner mode.

Acceptance criteria

• In multi-runner mode, typing .scenario offers available group keys
• In multi-runner mode, typing .scenario <group> offers scenario path/function completions scoped to that group
• Invalid groups do not crash completion and fall back gracefully
• In single-runner mode, existing .scenario <path> completions are preserved
• REPL tests assert completion behavior for both modes and nested scenario paths