chore(lint): adopt the canonical .golangci.yml from infra/ci-templates

Canonical lives at infra ci-templates/golangci.yml, distributed by the
infra#309 pattern: byte-identical copies, checksum drift gate in infra. Do
not edit this file here -- edit the canonical and run `task ci:sync`.

Verified locally before pushing, not pushed-and-hoped: golangci-lint v2.11.4
against a Go 1.26.1 toolchain (matching CI) reports 0 issues here.

Also applies gofumpt, which this config enables and the golangci-lint v2
default set does not check at all. That gap is why the drift was invisible:
earlier passes ran gofmt, which is weaker. Mechanical, deterministic,
build+vet verified.

internal/mcp/stdio_test.go

@@ -25,6 +25,7 @@ type echoTool struct{}
 func (echoTool) Descriptor() registry.ToolDescriptor {
 	return registry.ToolDescriptor{Name: "echo", Description: "echoes its input"}
 }
+
 func (echoTool) Call(_ context.Context, args json.RawMessage) (json.RawMessage, error) {
 	return args, nil
 }