Test suite is broken: restore runnable tests and add CI

## Problem

The Mocha + \`@vscode/test-electron\` test suite does not compile, so **no tests can run today**. The test build fails at the TypeScript step:

\`\`\`
$ yarn run build-test
node_modules/@types/markdown-it/lib/index.d.ts(151,33): error TS2694:
Namespace 'LinkifyIt' has no exported member 'LinkifyIt'.
\`\`\`

This is a transitive \`@types\` version drift: \`@types/markdown-it\` expects an older \`@types/linkify-it\` export shape than what currently resolves. Since `compile` (and therefore `yarn test`) depends on `build-test`, the entire suite is dark.

On top of that, two related gaps make the suite untrustworthy even once it compiles:

1. **Silent failure in the test runner.** \`src/test/runTest.ts\` catches any error, prints the generic string \`"Failed to run tests"\`, and calls \`process.exit(1)\` — but never prints the actual error. A broken run gives no diagnostic.
2. **No CI.** There is no \`.github/workflows/\`, so nothing runs the suite on push or PR. A test suite with no CI silently rots.

## Scope (Phase 0 — restore the framework)

- [ ] Fix the \`@types/markdown-it\` / \`@types/linkify-it\` type drift so \`tsc -p ./src/test\` compiles cleanly. (Likely the same \`markdown-it\` v14 bump flagged in Dependabot triage — coordinate with the dependency work.)
- [ ] Fix the swallowed error in \`src/test/runTest.ts\` so a failed run surfaces the real cause (fail loudly).
- [ ] Add a minimal GitHub Actions workflow running \`yarn install\`, \`yarn run build\`, and \`yarn test\` on push and PR.

## Out of scope (later phases)

- Adding test coverage for the markdownlint rule pack (AM001–AM029) — currently 0% covered.
- Migrating transform tests to a lighter runner (Vitest).
- These are tracked separately as Phase 1.

## Acceptance criteria

- \`yarn run build && yarn test\` completes locally with a visible pass/fail count.
- A failed test run prints the underlying error, not a generic message.
- CI runs the suite on every push and PR and reports status.

Start by reproducing the failure with yarn run build-test and inspect the dependency resolution behind the @types/markdown-it and @types/linkify-it mismatch. Read src/test/runTest.ts for the swallowed error and add the requested workflow under .github/workflows/. Done means yarn run build && yarn test passes with visible results, failures show their underlying error, and push and pull request CI runs the suite.

Written by the indexing model from the issue text.