Stabilize doctor command tests across local environments

[jjuanrivvera99]

Problem

TestDoctorCmd can fail in local runs because canvas doctor performs environment and network checks that depend on host state and local credentials.

Context

• During v1.8.0 release validation, full test runs passed under CI=1 but local doctor tests were environment-dependent.
• Current tests skip in CI, but local determinism is still weak.

Goal

Make doctor command tests deterministic and resilient while preserving useful coverage.

Suggested approach

• Add dependency injection or a mockable diagnostics runner in commands/doctor.go.
• Add focused unit tests for output formatting and failure handling without live network/filesystem dependencies.
• Keep one opt-in integration smoke test behind an explicit env var.

Acceptance criteria

• go test ./commands -run TestDoctorCmd is deterministic locally.
• No live network or host permission dependency by default.
• Runtime behavior of canvas doctor for users remains unchanged.