Use the canonical golangci-lint config (closes #66)

Replace `.golangci.yml` with the canonical file from `sneak/prompts`,
byte for byte. It runs `gomodguard_v2` in place of the deprecated
`gomodguard`, which removes the deprecation warning from every lint run,
and enables a `depguard` rule keeping `net/http/httptest` out of non-test
files. Neither raised a finding in this repo.

Model: opus-5-5

TODO.md

@@ -25,6 +25,12 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-10-04: `.golangci.yml` is again the canonical file from
+  `sneak/prompts`, byte for byte
+  (https://git.eeqj.de/sneak/secret/issues/66). It runs `gomodguard_v2`
+  in place of the deprecated `gomodguard`, so the lint no longer warns,
+  and enables `depguard` with a rule that keeps `net/http/httptest` out of
+  non-test files. Neither raised a finding in this repo.
 - 2026-10-04: `secret unlocker add pgp` works on Linux
   (https://git.eeqj.de/sneak/secret/issues/88). `CreatePGPUnlocker` gets
   the vault's long-term key as adding a passphrase unlocker does, with the