lint: adopt org-standard .golangci.yml and golangci-lint v2.12.2 (closes #14)

backend/.golangci.yml declared version: "2" on line 1 but used the
golangci-lint v1 schema below it: a top-level linters-settings key and
an issues.exclude-use-default key that does not exist in v2. Under v2
that config does not validate, so every threshold in it was inert --
lll fell back to its 120-column default rather than the intended 88,
and funlen, cyclop and dupl were not applied at all. The `0 issues.`
result the repo has been relying on was therefore meaningless.

Replace it with the org-standard file verbatim (sha256
021cc83f4e6fc7c31b95b34b846723dfcf20b66b7baeea1dc40406e643346bcb) and
repoint the golangci-lint pin in Dockerfile.backend from v2.7.2 to the
org-standard v2.12.2.

Guard against the config drifting from the standard again by asserting
its sha256 as the first step of the backend lint target. The check is a
local hash comparison against a constant in the Makefile: it needs no
network, fetches nothing, and adds no unpinned external reference to
the build path. It also catches a strictly larger class of breakage
than schema validation would, since a schema-valid but non-canonical
config is exactly how this file got into its broken state.

With the config actually loading, lll reports server.go:65 at 93
columns. Fix it, plus the two other over-long lines called out on the
issue (server.go:97 at 81 and reportbuf.go:166 at 88) which are inside
the 88-column lint limit but over the 77-column hard wrap in the Go
styleguide. All three were long //nolint justifications on the code
line; move the justification into a preceding comment block and leave
a short directive behind. No suppression is added or widened, and
.golangci.yml is not touched after the copy.

Drop the //nolint:wsl in server.go entirely rather than relocating it.
The standard config disables wsl, so the directive suppressed nothing;
removing it still yields `0 issues.`

Verified: `cd backend && make check` reports `0 issues.` and passes
with the network unavailable, root `make check` passes, and
`docker build -f Dockerfile.backend .` builds green against the pinned
v2.12.2.

backend/.golangci.yml

@@ -1,5 +1,9 @@
 version: "2"
 
+# Config schema uses the golangci-lint v2 layout (settings live under
+# linters.settings, not top-level linters-settings) so that the
+# thresholds below are actually applied by golangci-lint >= v2.
+
 run:
   timeout: 5m
   modules-download-mode: readonly
@@ -14,19 +18,17 @@ linters:
     - wsl              # Deprecated, replaced by wsl_v5
     - wrapcheck        # Too verbose for internal packages
     - varnamelen       # Short names like db, id are idiomatic Go
-
-linters-settings:
-  lll:
-    line-length: 88
-  funlen:
-    lines: 80
-    statements: 50
-  cyclop:
-    max-complexity: 15
-  dupl:
-    threshold: 100
+  settings:
+    lll:
+      line-length: 88
+    funlen:
+      lines: 80
+      statements: 50
+    cyclop:
+      max-complexity: 15
+    dupl:
+      threshold: 100
 
 issues:
-  exclude-use-default: false
   max-issues-per-linter: 0
   max-same-issues: 0

backend/Makefile

@@ -8,10 +8,20 @@ GOLDFLAGS += -X main.Buildarch=$(BUILDARCH)
 
 ifeq ($(UNAME_S),Darwin)
 	GOFLAGS := -ldflags "$(GOLDFLAGS)"
+	SHA256SUM := shasum -a 256
 else
 	GOFLAGS = -ldflags "-linkmode external -extldflags -static $(GOLDFLAGS)"
+	SHA256SUM := sha256sum
 endif
 
+# .golangci.yml is standardized org-wide and must never be edited here
+# (REPO_POLICIES.md). Its last silent drift replaced the v2 schema with
+# v1 keys, which left every threshold in the file inert while the build
+# stayed green. The lint target therefore asserts the file still matches
+# the canonical copy byte for byte. The check is a local hash comparison:
+# no network, no remote schema, nothing unpinned in the build path.
+GOLANGCI_CONFIG_SHA256 := 021cc83f4e6fc7c31b95b34b846723dfcf20b66b7baeea1dc40406e643346bcb
+
 .PHONY: all build test lint fmt fmt-check check docker hooks run clean
 
 all: build
@@ -25,6 +35,14 @@ test:
 	timeout 30 go test ./...
 
 lint:
+	@actual=$$($(SHA256SUM) .golangci.yml | cut -d' ' -f1); \
+	if [ "$$actual" != "$(GOLANGCI_CONFIG_SHA256)" ]; then \
+		echo ".golangci.yml has drifted from the org standard."; \
+		echo "  expected $(GOLANGCI_CONFIG_SHA256)"; \
+		echo "  actual   $$actual"; \
+		echo "Restore it verbatim from sneak/prompts; do not edit it."; \
+		exit 1; \
+	fi
 	golangci-lint run ./...
 
 fmt:

backend/internal/reportbuf/reportbuf.go

@@ -163,7 +163,9 @@ func (b *Buffer) writeFile(data []byte) {
 	name := fmt.Sprintf("reports-%s.jsonl.zst", ts)
 	path := filepath.Join(b.dataDir, name)
 
-	f, err := os.OpenFile( //nolint:gosec // path built from controlled dataDir + timestamp
+	// path is built from the operator-supplied dataDir plus a
+	// generated timestamp, so it carries no external input.
+	f, err := os.OpenFile( //nolint:gosec // see comment above
 		path,
 		os.O_WRONLY|os.O_CREATE|os.O_EXCL,
 		filePerms,

backend/internal/server/server.go

@@ -62,7 +62,10 @@ func New(
 		OnStart: func(_ context.Context) error {
 			s.startupTime = time.Now().UTC()
 
-			go func() { //nolint:contextcheck // fx OnStart ctx is startup-only; run() creates its own
+			// The fx OnStart context is scoped to startup and is
+			// cancelled once the hook returns; run() derives its
+			// own context instead of inheriting this one.
+			go func() { //nolint:contextcheck // see comment above
 				s.run()
 			}()
 
@@ -94,7 +97,7 @@ func (s *Server) run() {
 }
 
 func (s *Server) serve() int {
-	var ctx context.Context //nolint:wsl // ctx must be declared before multi-assign
+	var ctx context.Context
 
 	ctx, s.cancelFunc = context.WithCancel(
 		context.Background(),