Add read-only doctor diagnostics

[stef-k]

Summary

• add read-only human and JSON installation diagnostics through \code-guard doctor\
• report active distribution, Python, launcher, bundled skill, configuration, Git, and syntax-provider health
• preserve early CLI isolation, deterministic health/exit behavior, and existing analysis modes
• document troubleshooting usage and sensitive-output handling

Validation

• \python -m unittest tests.test_doctor\ (3 passed)
• \python -m unittest discover -s tests\ (294 passed)
• \git diff --check main...HEAD\
• Code Guard over the remediation files (PASS)

Closes #95