[codex] Add doctor live backend probes

[saworbit]

Summary

• Move orbit doctor into a command module and add repeatable --target <URI> live backend probes.
• Probe backend connectivity with a real list call, including env-backed backend config when ORBIT_BACKEND_TYPE is set.
• Add prefix-safe probe path handling for backends where parse_uri stores the URI path in the config.
• Document the feature in the changelog, getting started docs, quickstart guide, and v0.7 roadmap.

Validation

• cargo fmt
• cargo audit
• cargo clippy --lib --features backend-abstraction -- -D warnings
• cargo test --bin orbit doctor -- --nocapture
• cargo test --lib commands::doctor --all-features -- --nocapture

[codecov]

Codecov Report

❌ Patch coverage is 61.76471% with 91 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/commands/doctor.rs	62.28%	89 Missing ⚠️
src/main.rs	0.00%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Pass a real path for env-based SSH probes

When ORBIT_BACKEND_TYPE is set, the env probe always calls probe_config(..., None), and probe_path_for turns that into an empty PathBuf for non-prefix backends (including SSH). In the SSH backend, list() forwards that path to sftp.readdir(path), so orbit doctor can report a list/not-found error even with valid SSH credentials simply because no probe path was provided. This creates a false-negative health check specifically for env-driven SSH setups unless users also pass an explicit --target ssh://.../path.

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