Add status-badge generator (SVG + shields.io endpoint)

[cursor]

Summary

Add a status-badge generator: an SVG badge (total debt and/or trend arrow) teams can embed in a README, plus a shields.io-compatible JSON endpoint artifact.

Motivation

A visible badge is a cheap, high-impact adoption driver — it keeps maintainability debt in front of contributors on the repo landing page. DebtLens already produces all the numbers; it just needs a small badge reporter.

Proposed behavior

• debtlens scan --format badge (or debtlens badge) writes:
  • debtlens-badge.svg — a self-contained SVG showing e.g. "debt: 142" with severity-based color, and (when history data exists) a trend arrow.
  • debtlens-badge.json — a shields.io endpoint schema ({ schemaVersion, label, message, color }) so users can render via https://img.shields.io/endpoint?url=....
• Color thresholds configurable (green/yellow/red by total or by high-severity count).

Implementation surface

• New src/reporters/badgeReporter.ts (pure string SVG + JSON; no new dependency).
• Wire format/command into src/cli/commands/scan.ts (or a tiny src/cli/commands/badge.ts) and src/reporters/index.ts.
• Optional trend arrow reads .debtlens/history.jsonl if the history feature is present.
• Tests: SVG/JSON snapshot, color thresholds, shields endpoint schema validity.

Acceptance criteria

• Emits a valid self-contained SVG and a shields.io endpoint JSON.
• Color reflects configurable severity thresholds.
• Docs (README badge instructions) + CHANGELOG updated.
• npm run test:all passes.

Difficulty: small.