chore: add pre-commit config and update CI pipeline (#55)

## Summary
- New `.pre-commit-config.yaml` with standard Go hooks (gofmt, go vet, go mod tidy) plus file hygiene checks (trailing whitespace, end-of-file, yaml, large files, merge conflicts)
- go vet runs as a local hook with `./...` since the dnephin per-file hook doesn't work with Go module layouts
- Woodpecker pre-commit pipeline updated to use `almalinux9-gobuilder` image with `uvx pre-commit run --all-files`
- Pre-commit hooks installed into the repo

Reviewed-on: #55
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

.woodpecker/pre-commit.yaml

@@ -3,7 +3,15 @@ when:
 
 steps:
   - name: pre-commit
-    image: golang:1.25
+    image: git.unkin.net/unkin/almalinux9-gobuilder:20260606
     commands:
-      - test -z "$(gofmt -l .)"
-      - go vet ./...
+      - uvx pre-commit run --all-files
+    backend_options:
+      kubernetes:
+        resources:
+          requests:
+            memory: 512Mi
+            cpu: 1
+          limits:
+            memory: 2Gi
+            cpu: 2