feat: add quorum doctor command

[Solvely-Colin]

Summary

Adds a quorum doctor command that validates the user's Quorum setup in one command.

What it checks

1. Config — ~/.quorum/config.yaml exists and parses correctly
2. Node.js — version ≥20
3. Quorum version — current vs latest on npm
4. Providers — lightweight API probe (generates ~5 tokens) to verify auth works for each configured provider

Output format

✅ Config    ~/.quorum/config.yaml found and valid
✅ Node.js   v25.5.0 (requires ≥20)
✅ Quorum    v0.13.0 (latest)
✅ kimi      k2p5 — authenticated, 1598ms
❌ ollama    qwen2.5:14b — connection refused (is ollama running?)

4 healthy, 1 error

Error diagnosis

Provides actionable messages for common failures: 401 (bad key), 402 (no balance), ECONNREFUSED (service down), ENOTFOUND (DNS), timeouts.

Exit codes

• 0 — all checks pass (warnings ok)
• 1 — any errors

Tests

5 unit tests covering: all-pass, missing config, auth failure, connection refused, mixed results.

Files changed

• src/cli/doctor.ts — new command implementation
• src/cli/doctor.test.ts — tests
• src/cli/index.ts — register the command

[Solvely-Colin]

Clean implementation — good separation per check, parallel system checks, actionable error messages, sensible exit codes. Tests cover the important failure paths.

A few small things:

1. npm registry URL: https://registry.npmjs.org/quorum-ai/latest — confirm this is the published package name. If the npm package is named differently the version check will silently return a warn on every run.

2. package.json path: new URL("../../package.json", import.meta.url) works in src/ but verify the relative path holds in the compiled dist/ layout. A subtle breakage if the dist depth differs.

3. Provider probe cost: noted in the PR and acceptable for a diagnostic command — just worth a one-liner in the help text so users know it fires a real token call.

Otherwise looks good to merge.