test: add coverage reporting with regression thresholds

Add v8 coverage via a test:coverage script and vitest config: measure src
only, exclude the types-only deps.ts and the bin entrypoint main.ts, and
gate on ratcheted thresholds (statements 85, branches 78, functions 90,
lines 85) set just under current coverage. CI runs the unit+integration
tier through coverage so a real drop fails the build.

vitest.config.ts

@@ -6,5 +6,28 @@ export default defineConfig({
     // tier under test/e2e needs a live Gitea instance and runs via `test:e2e`.
     include: ["test/**/*.test.ts"],
     exclude: ["test/e2e/**"],
+    coverage: {
+      provider: "v8",
+      reporter: ["text", "html", "lcov"],
+      // Measure the shipped source only; the CLI is exercised through its seam,
+      // so coverage reflects what those tiers actually reach.
+      include: ["src/**/*.ts"],
+      exclude: [
+        // Types-only: no runtime code to exercise.
+        "src/deps.ts",
+        // Bin entrypoint: process argv/stdout/EPIPE wiring that delegates to the
+        // covered runCli seam; not reached by the in-process seam tests.
+        "src/main.ts",
+      ],
+      // A regression ratchet, not a target: set a few points under current
+      // coverage so a real drop fails CI while trivial churn does not. Raise
+      // these as coverage climbs; never lower them to make a red build pass.
+      thresholds: {
+        statements: 85,
+        branches: 78,
+        functions: 90,
+        lines: 85,
+      },
+    },
   },
 });