ci: add code coverage reporting and badge

[SebTardif]

What

Set up code coverage collection and add a coverage badge to the README.

Why

193 unit tests exist but no coverage metric is tracked. A coverage badge is a trust signal for quality and helps identify untested code paths.

Approach

1. Collect coverage using c8 (or node --test --experimental-test-coverage):

   npx c8 --reporter=text --reporter=json-summary node --test ./out-test/test/unit/*.test.js

2. Report in CI with one of:

   • Codecov: free for public repos, add codecov/codecov-action step
   • Self-hosted via gist: CI writes a shields.io endpoint JSON to a gist (no vendor dependency, per repo-badges skill lesson Add diff preview and quick actions for common Patchloom workflows #8)

3. Add badge to README:

   [![codecov](https://codecov.io/gh/patchloom/patchloom-vscode/branch/main/graph/badge.svg)](https://codecov.io/gh/patchloom/patchloom-vscode)

Per badge hygiene rule #3, only add the badge after coverage is at a reasonable level.

Identified by repo-badges skill audit.