cli: clearer PASS/FAIL/SKIPPED summary line for mutiny test

[CodewithJha]

Problem

mutiny test prints per-case lines and counts (test_cmd.py), but GOOD_FIRST_ISSUES #22 asks for a friendlier single summary line. Easy DX win for CI logs.

Repository evidence

• packages/mutiny_cli/src/mutiny_cli/test_cmd.py — aggregates passed / failed / skipped
• Catalog policy: support dotted paths for nested tool arguments #22

Proposed direction

Always print one machine-scannable summary, e.g. Summary: 3 passed, 1 failed, 0 skipped (match Hosted tests page tone). Keep --json as structured output.

Acceptance criteria

• Summary line always shown for non-json human mode when cases ran
• Unit test asserts substring when capturing stdout
• docs/CLI.md example updated if needed

Difficulty

XS · CLI · good first issue