chore: update golangci-lint to v2.12.2 with canonical config (#54)

Canonical v2-schema `.golangci.yml`, golangci-lint pins bumped to v2.12.2 in `Dockerfile` and `script/bootstrap`, and the tree brought to `0 issues.` under it.

Three behaviour deltas: `Cache.StoreVariant` takes a context (cancelled requests skip the accounting row, recovered by reconciliation); `MetadataStorage.Store` no longer leaks `.tmp-*.json` on Write/Close/Rename failure (dead-defer bug fix); the `signing_key` too-short error text gained a `value too short:` prefix.

Eviction-loop context cancellation deferred to #102.

TODO.md

@@ -23,6 +23,27 @@ P1: implement blocked networks configuration to extend SSRF protection
 
 # Completed Steps
 
+- 2026-08-07 update golangci-lint to v2.12.2 with the canonical
+  `.golangci.yml` (v2 schema, `default: all` minus six disabled
+  linters, `lll` 88, tests included): bumped the pinned
+  `golangci/golangci-lint:v2.12.2-alpine` image in `Dockerfile` and the
+  release-archive sha256 pins in `script/bootstrap`; fixed the findings
+  the stricter config surfaced (notably `paralleltest`, `wsl_v5`,
+  `goconst`, `lll`, `noinlineerr`, `err113`, `errcheck`, `testpackage` —
+  white-box test files renamed to `*_internal_test.go`), including #55's
+  code absorbed after it merged, iterating the pinned linter to
+  `0 issues.`; no single finding total is substantiable, since
+  golangci-lint's `uniq-by-line` reveals new findings on a line as
+  others there are fixed — the documented re-measurements were 81 after
+  the #53 merge and 149 after the #55 merge; three behavior changes, so
+  not a pure no-op: `Cache.StoreVariant` now takes a `context.Context`
+  (`noctx`), so a cancelled request skips its best-effort accounting
+  row; `MetadataStorage.Store`'s cleanup defer was dead on `main` and
+  leaked `.tmp-*.json` on failure, now fixed with explicit removals; and
+  the `signing_key` validation error text gained `value too short: `;
+  the eviction loop's uncancellable context is deferred to #102 under a
+  `//nolint:contextcheck`; three `//nolint:tagliatelle` directives keep
+  the snake_case JSON wire/disk formats unchanged; `make check` green
 - 2026-08-07 implement cache size management and eviction (closes
   #51): new `cache_max_bytes` config key validated by the startup
   framework (explicit values used exactly with no floor, `0` disables