feat(doctor): add --json output for machine-readable diagnostics

[alimaandev]

ripple doctor currently prints a human report only. Add --json so CI and tools can consume the diagnostics programmatically.

Acceptance criteria

• ripple doctor --json emits the same fields as the terminal report (config path, cache status, discovery stats, errors) in the stable JSON envelope (tool, version, command, ...) used by analyze/graph.
• A --format terminal|json flag (alias -f) matching the other commands, with --format in the help text.
• JSON output is byte-deterministic for the same project (no timestamps beyond durationMs).

Where to look

• src/commands/doctor.ts (current report), src/formatter/ (existing formatters to copy the envelope from), tests/integration/cli.test.ts (test pattern for other --format json commands).
• Existing style: see src/formatter/json.ts.

Out of scope

• Changing what doctor checks.