Update golangci-lint to v2.12.2 with canonical config (#62)

Updates golangci-lint to v2.12.2 everywhere it is pinned and installs the canonical `.golangci.yml`, then remediates every finding the new linter/config surfaces so `make check` is green.

## Version bump

- `Dockerfile` lint stage: `golangci/golangci-lint:v2.11.3-alpine` -> `v2.12.2-alpine` (digest-pinned, date comment updated)
- `Makefile` `deps` target: `go install` moved from the old v1 module path at `@latest` to the pinned `github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.12.2`
- `.golangci.yml` replaced with the canonical config (v2 schema; settings under `linters.settings` so the thresholds actually apply; `default: all` with the standard six disables)
- `script/bootstrap` installs golangci-lint via the system package manager and carries no version pin, so it is unchanged
- CI (`.gitea/workflows/check.yml`) only runs `script/cibuild`, so it needed no change

## Lint remediation

The canonical config surfaced ~3,300 findings across 56k lines. All are fixed, behavior-preserving; incorporates and supersedes the per-package mechanical passes already merged to `main` (refs #61). Highlights:

- `err113`: dynamic errors replaced with package sentinels + `%w` wrapping; comparisons via `errors.Is`
- `goprintffuncname`: printf-style helpers renamed with an `f` suffix (`ui.Writer` message methods, `cli.ReportErrorf`, `database.Fatalf`) and all call sites updated
- `revive` stutter renames: `blob.Handler`, `blob.WithReader`, `blob.ChunkPosition`, `storage.URL`, `storage.Info`; missing doc comments added
- `contextcheck`/`noctx`: `context.Context` threaded through `blob.Packer` and the scanner call sites; context-aware `exec`/`sql` variants
- `funlen`/`cyclop`/`gocognit`/`dupl`: oversized and duplicated functions split into focused helpers (production and test code)
- tests: `t.Parallel()` added where safe (global logger init kept in the serial phase for `-race`), `t.TempDir()`/`t.Helper()` adopted, several suites converted to external test packages
- `gosec`: bounded integer conversions, `ReadHeaderTimeout` on the test HTTP server; remaining warnings suppressed per-site with justifications
- remaining `nolint` directives are rare, targeted, and each carries a reason (e.g. `nilnil` not-found contract in the repository layer, fx module globals, on-disk snake_case struct tags)
- removed the deprecated `log.LogOptions` alias (callers migrated to `log.Options`)

`make check` (tests with `-race`, lint, fmt-check) passes.

Co-authored-by: sneak <sneak@sneak.berlin>
Reviewed-on: #62
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/chunker/chunker_isolated_test.go

@@ -1,11 +1,15 @@
-package chunker
+package chunker_test
 
 import (
 	"bytes"
 	"testing"
+
+	"sneak.berlin/go/vaultik/internal/chunker"
 )
 
 func TestChunkerExpectedChunkCount(t *testing.T) {
+	t.Parallel()
+
 	tests := []struct {
 		name         string
 		fileSize     int
@@ -38,16 +42,19 @@ func TestChunkerExpectedChunkCount(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			chunker := NewChunker(tt.avgChunkSize)
+			t.Parallel()
+
+			c := chunker.NewChunker(tt.avgChunkSize)
 
 			// Create data with some variation to trigger chunk boundaries
 			data := make([]byte, tt.fileSize)
 			for i := range data {
 				// Use a pattern that should create boundaries
+				//nolint:gosec // G115: intentional byte truncation
 				data[i] = byte((i * 17) ^ (i >> 5))
 			}
 
-			chunks, err := chunker.ChunkReader(bytes.NewReader(data))
+			chunks, err := c.ChunkReader(bytes.NewReader(data))
 			if err != nil {
 				t.Fatalf("chunking failed: %v", err)
 			}