fix: prevent false link-check failures and harden health/workflow errors

Address three review findings from the Go rewrite:\n\n- checker: update PartitionLinks to only classify HTTP(S) URLs as external links.\n  This skips markdown-relative targets (for example  and anchors) so\n  non-URL entries are no longer sent to HTTP validation and do not produce\n  deterministic "unsupported protocol scheme" failures.\n\n- health command: stop ignoring LoadHealthCache errors.\n  Return a user-facing "load cache" error when the cache file is unreadable\n  or invalid instead of allowing a nil cache panic on Merge.\n\n- broken links workflow: remove masked execution behavior from the link check\n  step. Capture awesome-docker check exit code, set has_errors=true on any\n  non-zero exit, and expose the exit code in the generated issue body so\n  checker failures are visible and cannot incorrectly close the tracking issue.\n\nTest coverage updates:\n- extend checker partition test to include markdown-relative/anchor targets\n  and verify they are not treated as external URLs.\n- add cache test for invalid YAML load failure.

internal/cache/cache_test.go

@@ -78,6 +78,22 @@ func TestLoadHealthCacheMissing(t *testing.T) {
 	}
 }
 
+func TestLoadHealthCacheInvalidYAML(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "health.yaml")
+	if err := os.WriteFile(path, []byte("entries:\n  - url: [not yaml"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	hc, err := LoadHealthCache(path)
+	if err == nil {
+		t.Fatal("expected error for invalid YAML")
+	}
+	if hc != nil {
+		t.Fatal("expected nil cache on invalid YAML")
+	}
+}
+
 func TestMerge(t *testing.T) {
 	hc := &HealthCache{
 		Entries: []HealthEntry{