Add JSON output for doctor diagnostics

[delaudio]

Parent epic: #31

Problem

Integrations need to show actionable diagnostics without scraping the compact doctor table and prose.

Scope

Add:

hum <project> <template> doctor --json

Represent project-level, task-level, and service-level checks as structured records with stable status and issue identifiers.

Acceptance criteria

• doctor --json uses the common versioned envelope.
• Each check includes scope, subject, label or code, pass/fail state, and safe detail.
• The response includes aggregate totals and an overall result.
• Failed checks retain a non-zero process exit code.
• Task failures are represented as data rather than corrupting stdout.
• Secret values and raw provider errors are redacted.
• Ordering and field semantics are documented and fixture-tested.
• Existing doctor table output remains unchanged without --json.