Add sp doctor --json output for scripting

[rwrife]

Advances #6 (M6 acceptance: "--json output mode for ls (and other read commands) for scripting").

sp ls already had --json; sp doctor produces the richest structured data in the tool (health flag, counts, footprint, orphan/missing drift) but had no machine-readable mode. This adds it.

What changed

• internal/render/json.go — new DoctorJSON view + DoctorReportJSON(w, data) emitter. Mirrors the human report's information as pure data: top-level healthy flag, live/morgue counts, tracked/orphan/total sizes (raw bytes and human strings, matching the ls JSON convention), and orphans/missing arrays. Slices are always arrays, never null, so scripts can iterate unconditionally. Color- and personality-free, same contract as sp ls --json.
• internal/cli/doctor.go — --json flag routes to the JSON emitter and, like ls --json, never tints output regardless of TTY/--no-color.
• Tests — render-layer unit tests (healthy + drift) and full-command CLI tests (doctor_json_test.go) asserting valid parseable JSON, colorless/flavor-free output, correct healthy flip, orphan/missing surfacing, and that the run stays read-only (orphan still on disk afterward).
• Docs — README doctor section + quick-reference updated (sp doctor --json | jq -e '.healthy'); root.go package doc updated.

Example

{
  "healthy": false,
  "liveCount": 1,
  "morgueCount": 0,
  "trackedSize": 0, "trackedSizeHuman": "0B",
  "orphanSize": 5,  "orphanSizeHuman": "5B",
  "totalSize": 5,   "totalSizeHuman": "5B",
  "orphans": [ { "path": ".../scratches/cafef00d.md", "area": "scratches", "size": 5, "sizeHuman": "5B" } ],
  "missing": []
}

Verification

go build ./..., go vet ./..., go test ./... all green; go mod tidy clean; gofmt clean.

Not in scope (remaining M6)

Tagging/publishing v0.1.0 and pointing the README install section at the release are deliberately left out — the release workflow is tag-driven and a maintainer decision.