chore: project hardening — dep swap, CI gates, dependabot, action bumps (#8)

* chore: project hardening — dep swap, CI gates, dependabot, action bumps

- swap gopkg.in/yaml.v3 (upstream archived Apr 2025) for maintained
  github.com/goccy/go-yaml; same Unmarshal API, tags unchanged
- add CI workflow: go vet/test/build + golangci-lint + govulncheck
  on PRs and main pushes
- add dependabot for gomod + github-actions (weekly)
- bump actions to latest majors in update.yml (clears Node 20
  deprecation annotations); workflow logic untouched
- README/history refreshed by E2E verification run (29 agents)

* fix: check Close/Remove error returns (errcheck)

Write paths (writeSnapshots, renderReadme) now propagate close errors —
a failed close there can hide lost data. Read/cleanup paths ignore
explicitly with _ =.

.github/dependabot.yml

@@ -0,0 +1,13 @@
+version: 2
+updates:
+  - package-ecosystem: "gomod"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5