Step 22 C: metrics over the registry, documentation, PR-doc

The metrics page sums the registered repositories' sizes; the instance
configuration is documented in docs/configuration.md, the architecture skill,
AGENTS.md and the step 22 plan (session C ticked, fairness decided FIFO).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/test/kotlin/de/hoennig/werkator/metrics/SystemMetricsCollectorTest.kt

@@ -20,7 +20,7 @@ class SystemMetricsCollectorTest : FunSpec() {
         repoSizeBytes: (Path) -> Long = { HALF_GIB_BYTES },
     ) = SystemMetricsCollector(
         stateFile = { tempDir.resolve("system-metrics-state.json") },
-        workingDir = tempDir,
+        repoDirs = { listOf(tempDir) },
         clock = Clock.fixed(now, ZoneOffset.UTC),
         procStat = tempDir.resolve("stat"),
         procMeminfo = tempDir.resolve("meminfo"),