Migrate canonical .golangci.yml to gomodguard_v2, with a block list (#55)

Option-2 answer to sneak's ruling of 2026-08-19 on
sneak/homoicon#4: migrate to the successor, with
settings. Closes #25.

## The change

`golangci-lint` v2.12.0 deprecated `gomodguard`, and this config sets
`linters.default: all`, so it is enabled everywhere and warns on every run.

- `gomodguard` joins `wsl` in `linters.disable` under a shared "deprecated"
  comment. The warning is attached to the old name, so disabling it is what
  silences it.
- `gomodguard_v2` is named in `linters.enable`, a no-op under `default: all`
  that gives the settings block a visible owner.
- Blocked, each restating a decision already recorded in the Go package
  defaults: `rs/zerolog` → `log/slog`; the pre-fork `go-redis/redis` →
  `redis/go-redis/v9`; `sergi/go-diff` and `hexops/gotextdiff` → `go-udiff`.
  Every entry matches the module path exactly, so the pre-fork go-redis takes
  three: `go-redis/redis`, `/v7`, `/v8`. A prefix would also cover
  `go-redis/redismock`, the test double for the successor recommended here.

Deliberately absent: recorded rejections that vendoring repos still require
(`mattn/go-sqlite3`, `gorm.io/gorm`, `pmezard/go-difflib`), plus `urfave/cli`
and unversioned `go-chi/chi`. Blocking those would redden repos mid-migration
on their next re-vendor.

## After merge

The file's sha256 moves from
`d10f47ef5e0d8620efd62275b016a7de8fd5abedf333922eec86fe4abc06176e` to
`a79b63a254602a5318db5d0e9a06bc71b84bf0c1d896305229d8bfed1d1b1776`, so every
vendoring repo mismatches. #60 is the
propagation brief; it and the record on
#25 carry this value.

## Disclosures

- Judgement call: `wsl` moved two lines down to share the "deprecated" comment.
  No behaviour change, but it widens the diff.
- The `go.mod` survey and the settings-block probe are on
  #25.
- Unverified: the linter was not run against each vendoring repo; the per-repo
  claim rests on reading their `go.mod` files.
- `make check` exit 0.

Model: opus-5
Co-authored-by: Jeffrey Paul <sneak@noreply.example.org>
Reviewed-on: #55
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

TODO.md

@@ -57,6 +57,11 @@ fmt-check, and commit.
   `make test` example in `REPO_POLICIES.md`, so the target cannot report a
   cached pass it did not earn, and documented that Go's test-result cache is a
   second, independent cache stacked below the Docker layer cache.
+- 2026-08-31: Migrated the canonical `.golangci.yml` from the deprecated
+  `gomodguard` to `gomodguard_v2`: the old linter is disabled by name (which is
+  what silences the deprecation warning), the successor is named explicitly in
+  `linters.enable`, and it carries a `blocked` module list drawn only from
+  decisions already recorded in the Go package defaults.
 - 2026-08-07: Set the canonical `.golangci.yml` to the org-standard v2-schema
   config already deployed byte-identical across the org's Go repos (settings
   under `linters.settings` so thresholds like lll/funlen/cyclop/dupl actually