Add `eval-harness doctor` — preflight diagnostics command

[hoainho]

Why

Today, when something goes wrong (missing jq, wrong Python, opencode not on PATH, API key not set, registry not initialized), the user gets a cryptic error somewhere downstream. A doctor subcommand that prints a green/red checklist of every prerequisite would cut first-run friction enormously.

What to build

A new scripts/eval/doctor.sh invoked as eval-harness doctor that checks and prints:

eval-harness doctor

[✓] bash 5.2.15
[✓] jq 1.7.1
[✓] python3 3.11.4 + yaml module
[✓] opencode 1.15.2 on PATH
[✓] ANTHROPIC_API_KEY set (sk-ant-...e4f2)
[✗] OPENCODE_SKILLS_ROOT not set — will walk up from cwd
[✓] registry initialized at /Users/x/.eval-harness/state/registry.json
[✓] this repo is enabled in registry
[✓] pricing.json fresh (12 days old, max 60)

Status: 8/9 OK. See https://github.com/nano-step/eval-harness/blob/main/docs/concepts.md for context on warnings.

Exit codes:

• 0 if all checks pass (warnings allowed)
• 64 if any check fails (missing required tool)

Acceptance criteria

• New scripts/eval/doctor.sh (set -euo pipefail)
• Wired into run.sh via a doctor subcommand
• Each check is its own function so they're easy to extend
• Test at scripts/eval/tests/doctor.sh covering: all pass, one fail, one warning
• README quickstart section gets a "When in doubt, run eval-harness doctor" line

Good first issue because

• Self-contained new file
• No coupling to score / attribute / stability
• Pattern mirrors existing preflight.sh which can be cannibalized for check helpers
• 100-150 lines of bash

Comment to claim.