feat: add pythonlings doctor diagnostics

[abhiksark]

Motivation

A read-only diagnostic command can turn opaque setup problems into actionable checks for learners and maintainers without changing user workspaces.

Current evidence

• The existing product-hardening umbrella (Product-hardening umbrella #4) identifies pythonlings doctor as a desired diagnostic command.
• Workspace validity currently spans CLI root resolution, info.toml, mirrored curriculum files, state, reset snapshots, solutions, and bundled docs.
• Existing state loading can mutate corrupt files, so doctor needs its own non-mutating inspection path.

Exact scope

Add pythonlings [--root PATH] doctor, a read-only diagnostic command that reports runtime/package version, workspace structure, manifest, solutions, state, reset snapshots, and bundled docs. Warnings do not fail the command; required failures return 1.

Acceptance checklist

• The command is discoverable in CLI help and respects global --root.
• It prints clear status for Python runtime and installed package version.
• It checks required workspace structure, manifest validity, solution availability, state readability, reset snapshots, and bundled docs.
• It distinguishes warnings from required failures, uses actionable messages, and exits 0 when only warnings occur.
• A required failure exits 1; malformed/missing manifest handling remains friendly and never prints a traceback.
• The command does not create, repair, rename, or otherwise modify workspace files.
• Unit and CLI integration tests cover healthy, warning-only, required-failure, malformed-state, and --root paths.

Explicit exclusions

Do not add automatic repair, network calls, telemetry, a TUI screen, or a release promise. Do not reuse a mutating load path in the read-only diagnostics command.

Likely files

• pythonlings/cli.py
• pythonlings/core/doctor.py (new)
• pythonlings/core/manifest.py, state.py, reset.py, or docs helpers as needed
• tests/unit/test_doctor.py (new)
• tests/integration/test_cli_doctor.py (new)

Verification

python -m pytest tests/unit/test_doctor.py tests/integration/test_cli_doctor.py -q
python -m pytest -q
pythonlings --root tests/fixtures/passing_curriculum doctor

Compare workspace file hashes before and after doctor to demonstrate read-only behavior.

Sprint coordination

See the sprint Discussion. This issue is unassigned and is not a reservation.