chore: add golangci-lint, bump Go to 1.26, fix all lint issues (#133)

## Summary
- Add `.golangci.yml` with linter configuration matching the main gitea repo
- Add `lint`, `lint-fix`, `lint-go`, `lint-go-fix`, and `security-check` Makefile targets
- Add `tidy` Makefile target (extracts min Go version from `go.mod` for `-compat` flag)
- Bump minimum Go version to 1.26
- Update golangci-lint to v2.10.1
- Replace `golang/govulncheck-action` with `make security-check` in CI
- Add `make lint` step to CI
- Fix all lint issues across the codebase (formatting, `errors.New` vs `fmt.Errorf`, `any` vs `interface{}`, unused returns, stuttering names, Go 1.26 `new(expr)`, etc.)
- Remove unused `pkg/ptr` package (inlined by Go 1.26 `new(expr)`)
- Remove dead linter exclusions (staticcheck, gocritic, testifylint, dupl)

## Test plan
- [x] `make lint` passes
- [x] `go test ./...` passes
- [x] `make build` succeeds

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/133
Reviewed-by: techknowlogick <techknowlogick@noreply.gitea.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

.gitea/workflows/test-pr.yml

@@ -11,16 +11,9 @@ jobs:
       - uses: actions/setup-go@v5
         with:
           go-version-file: 'go.mod'
+      - name: lint
+        run: make lint
       - name: build
-        run: |
-          make build
-
-  govulncheck_job:
-    runs-on: ubuntu-latest
-    name: Run govulncheck
-    steps:
-      - id: govulncheck
-        uses: golang/govulncheck-action@v1
-        with:
-          go-version-file: 'go.mod'
-          go-package: ./...
+        run: make build
+      - name: security-check
+        run: make security-check