feat(doctor): add diagnostic result model and base checks

[Karib0u]

Purpose

Create the reusable diagnostic model needed by rustinel doctor and later by rustinel setup health checks.

Scope

• Add a common diagnostic result model.
• Add human-readable doctor output.
• Add doctor --json for automation.
• Implement exit codes:
  • 0 healthy
  • 1 warnings
  • 2 failures
• Detect portable versus managed mode.
• Check configuration discovery and parsing.
• Report resolved paths.
• Check log and alert directory writability where applicable.
• Aggregate pass, warn, and fail results consistently.
• Keep the command strictly read-only.

Acceptance Criteria

• rustinel doctor runs without starting the full agent.
• doctor --json returns structured results suitable for automation.
• Exit codes distinguish healthy, warning, and failure states.
• Portable versus managed mode is reported.
• Configuration discovery and parse failures produce actionable output.
• Result aggregation is covered by tests.
• Doctor remains read-only.

Dependencies

Depends on the layout and configuration foundation for full path reporting, but the result model can start independently.

Priority

P0