chore(backend): re-vendor .golangci.yml with gomodguard_v2 (closes #41)

golangci-lint v2.12 deprecates gomodguard, which the org .golangci.yml
reached through "default: all", so every lint run printed a
deprecation warning. backend/.golangci.yml is now the current copy
from sneak/prompts, fetched unedited: gomodguard is disabled and
gomodguard_v2 enabled with the org block list. The new file also
turns depguard on with its test-support rule, which forbids
net/http/httptest outside test code. netwatch has no test-support
packages of its own to add to that rule, so the file is identical to
the canonical one. backend/script/lint checks the new sha256. The
backend raises no findings under the new rules.

Model: opus-5-5

TODO.md

@@ -23,6 +23,13 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: `backend/.golangci.yml` re-vendored from `sneak/prompts` (issue
+  #41): `gomodguard`, deprecated in golangci-lint v2.12.0, is disabled and its
+  successor `gomodguard_v2` enabled with the org block list, so lint runs print
+  no deprecation warning. The new file also turns `depguard` on with its
+  `test-support` rule, which keeps `net/http/httptest` out of non-test code;
+  netwatch adds no entries of its own to that rule. `backend/script/lint` checks
+  the new sha256
 - 2026-09-29: nginx sends the security headers `REPO_POLICIES.md` requires on
   every response (issue #18), including errors, `/assets/` and what it passes on
   from the backend, whose own copies it drops so each header goes out once. They
@@ -191,9 +198,3 @@ latest run passes.
   (main always green policy)
 - Decide what to do with untracked resume.sh: commit it, gitignore it, or delete
   it
-- Upstream fix needed in `sneak/prompts`: the org-standard `.golangci.yml`
-  enables `gomodguard`, which golangci-lint v2.12.2 reports as deprecated since
-  v2.12.0 and replaced by `gomodguard_v2`, so every backend lint run prints a
-  deprecation warning. The file is standardized and must never be edited in this
-  repo, so nothing can be done here beyond tracking it — tracked at
-  <https://git.eeqj.de/sneak/netwatch/issues/41>