Fix Ruff baseline errors in src/pet/cli.py

[gcomneno]

Track and fix existing Ruff baseline errors in src/pet/cli.py.

Observed on main after PR #135:

• F401 unused import: heapq
• F401 unused import: collections.Counter
• F401 unused import: collections.deque
• F401 unused import: minimal_shape_representative
• F401 unused import: shape_generator_from_factorization
• F821 undefined name: to_jsonable

Scope:

• Keep this separate from guarded redirect work.
• Fix warnings surgically.
• Avoid broad formatting or large CLI refactors.
• Verify whether to_jsonable is a real bug, dead code, or missing import/helper.
• Add or run targeted tests for touched CLI paths.

Boundary:

• No large src/pet/cli.py reformat.
• No unrelated CLI behavior changes.
• No broad refactor unless split into a separate issue.

[gcomneno]

Fixed on main in 94895a9.

Summary:

• Removed unused imports reported by Ruff in src/pet/cli.py.
• Imported to_jsonable from pet.io for the existing JSON path.
• Kept the patch surgical: no broad CLI formatting or refactor.

Validation:

• uv run ruff check src/pet/cli.py
• python -m py_compile src/pet/cli.py
• uv run pytest -q tests/test_cli_structural_factorization.py tests/test_cli_structural_route.py -rs