Update golangci-lint to v2.12.2 with canonical config (closes #30)

- Replace .golangci.yml with the canonical strict config (all linters
  enabled except the standard disable list; lll 88, funlen 80/50,
  cyclop 15, dupl 100; test files now linted)
- Pin the Dockerfile lint stage to golangci/golangci-lint:v2.12.2 by
  tag and digest (Debian-based)
- Fix all ~1550 findings surfaced by the new config: line wrapping,
  wsl_v5/nlreturn blank lines, noinlineerr splits, err113 sentinel
  errors, perfsprint/modernize rewrites, goconst constants, thelper,
  testifylint, noctx CommandContext, testpackage conversions,
  t.Parallel() where safe, and complexity/dupl helper extraction
- Record the change and follow-up items in TODO.md

User-visible strings
--------------------

No user-visible string changes remain. Every error message this branch
composes is byte-identical to the one main composes.

The err113 sentinels are shaped so that fmt.Errorf reassembles the
original text around them: a sentinel carries the fixed words of the
message and the caller supplies the interpolated value in the position
it has always occupied. Where the value sits in the middle of the
sentence the sentinel therefore holds only a fragment (for example
vault.ErrVaultNotFound is "does not exist", composed by its caller as
"vault <name> does not exist"); each such sentinel documents the
message it participates in.

Verified mechanically rather than by inspection: every fmt.Errorf and
errors.New call site in both trees was parsed, the Error() text of any
sentinel passed to %w substituted in, and the resulting sets of
composed message templates compared. All 350 templates main produces
are still produced, character for character; the set of messages lost
or altered is empty.

unlocker list
-------------

findUnlockerIDByMetadata now returns (string, error) instead of
signalling failure with an empty ID. An unreadable unlockers.d is no
longer indistinguishable from "no matching entry", so UnlockersList
skips the entry with a warning naming the directory, as it did before
the scan was extracted into a helper, rather than emitting a row under
a synthesized fallback ID that no unlocker remove or unlocker select
can match and that suppresses the current-unlocker marker. The
duplicate-check and shell-completion callers skip on the same
condition, matching their pre-extraction behavior. Covered by tests in
internal/cli/unlockers_list_test.go.

TODO.md

@@ -25,6 +25,20 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-08-07: Updated golangci-lint to v2.12.2 with the canonical
+  `.golangci.yml` (all linters enabled minus the standard disable
+  list, `lll` 88, tests linted); bumped the `Dockerfile` lint-stage
+  image to the tagged v2.12.2 Debian digest; fixed all ~1550 new
+  findings across `internal/` and `pkg/` (line wrapping, `wsl_v5`
+  blank lines, sentinel errors for `err113`, `t.Parallel()` where
+  safe, `_test` package conversions, complexity/`dupl` helper
+  extraction) on branch `golangci-v2.12.2`. Reworked after review:
+  the `err113` sentinels in `internal/vault`, `internal/secret`,
+  `internal/cli` and `pkg/bip85` were reshaped so every composed
+  error message is byte-identical to `main`, and
+  `findUnlockerIDByMetadata` now returns an error so `unlocker list`
+  skips an unreadable `unlockers.d` entry with a warning instead of
+  emitting a fabricated fallback ID.
 - 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints,
   Makefile shims, README Entrypoints section
 - 2026-03-11: Secure Enclave unlocker for hardware-backed secret
@@ -50,6 +64,17 @@ Bring the repo into policy compliance in one commit:
 
 - Compliance (after Next Step lands): keep main green under the new
   .gitea workflow; run make check before every merge.
+- Implement version-number shell completion for the second arg of
+  `secret version promote` and `secret version rm`
+  (`internal/cli/version.go`; was an in-code TODO removed for godox).
+- Cover mnemonic-vs-xprv identity consistency in
+  `pkg/agehd/agehd_test.go` `TestMnemonicVsXPRVConsistency` (was an
+  in-code FIXME removed for godox).
+- Darwin-gated files (`internal/secret/keychainunlocker.go`,
+  `seunlocker_darwin.go`, `internal/macse/macse_darwin.go`, related
+  tests) are not linted on the Linux CI runner and still contain lines
+  over the new 88-column limit; they will surface if lint ever runs on
+  macOS.
 - Merge secure-enclave-unlocker to main once review is done.
 - 1.0 critical security blockers (from repo TODO.md):
     - Command injection: GPG key IDs passed unescaped to exec.Command