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

The old backend/.golangci.yml declared version "2" but used v1 schema
keys, so under v2 it never validated and its thresholds were inert: the
linter ran at defaults. Replace it verbatim with the org-standard file,
repin the Dockerfile.backend lint stage from golangci-lint v2.7.2 to
v2.12.2, and assert the config's sha256 as the first step of the backend
lint target so it cannot silently drift again -- a local hash check
against a constant, no network. With the config loading, lll flags one
over-long line; wrap it and the two others named on the issue, moving
each //nolint justification above the code line, and drop a dead
//nolint:wsl (wsl is disabled by the standard config). TODO.md updated.

Model: opus-4-8

backend/Makefile

@@ -10,6 +10,17 @@ GOLDFLAGS += -s -w
 GOLDFLAGS += -X main.Version=$(VERSION)
 GOLDFLAGS += -X main.Buildarch=$(BUILDARCH)
 
+# macOS ships shasum rather than sha256sum.
+SHA256SUM := $(shell command -v sha256sum >/dev/null 2>&1 && echo sha256sum || echo shasum -a 256)
+
+# .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
@@ -22,6 +33,14 @@ 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: