build: update golangci-lint to v2.12.2 with org-standard v2 config

Pin golangci-lint to commit c0d3ddc9cf3faa61a4e378e879ece580256d76e5
(v2.12.2) in Dockerfile and script/bootstrap.

Set .golangci.yml to the org-standard v2-schema config already used
across the org's repos (owner-authorized; the same file is being
landed as canonical via a prompts-repo PR). Lint settings live under
linters.settings, so the lll, funlen, cyclop, and dupl thresholds are
actually applied. The informational gomodguard deprecation warning
this config can emit under v2.12 is accepted.

Fix all findings surfaced by the now-active thresholds:

- goconst: shared constants for repeated status, priority, and DNS
  fixture strings in watcher.go and the notify, state, and watcher
  tests
- dupl: consolidate duplicated ntfy/slack HTTP-error tests and
  SendNotification endpoint-error tests behind shared helpers
- lll: wrap long test table entries and comments; shorten one inline
  nolint justification

.golangci.yml

@@ -1,5 +1,9 @@
 version: "2"
 
+# Config schema uses the golangci-lint v2 layout (settings live under
+# linters.settings, not top-level linters-settings) so that the
+# thresholds below are actually applied by golangci-lint >= v2.
+
 run:
   timeout: 5m
   modules-download-mode: readonly
@@ -12,7 +16,6 @@ linters:
     - depguard         # Dependency allow/block lists
     - godot            # Requires comments to end with periods
     - wsl              # Deprecated, replaced by wsl_v5
-    - gomodguard       # Deprecated, replaced by gomodguard_v2
     - wrapcheck        # Too verbose for internal packages
     - varnamelen       # Short names like db, id are idiomatic Go
   settings:
@@ -25,25 +28,7 @@ linters:
       max-complexity: 15
     dupl:
       threshold: 100
-  exclusions:
-    generated: lax
-    paths:
-      - third_party$
-      - builtin$
-      - examples$
 
 issues:
   max-issues-per-linter: 0
   max-same-issues: 0
-
-formatters:
-  enable:
-    - gofmt
-    - gofumpt
-    - goimports
-  exclusions:
-    generated: lax
-    paths:
-      - third_party$
-      - builtin$
-      - examples$

TODO.md

@@ -26,11 +26,13 @@ confirm make check still passes.
 # Completed Steps
 
 - 2026-08-07: golangci-lint bumped to v2.12.2 (commit-pinned installs
-  in `Dockerfile` and `script/bootstrap`); `.golangci.yml` migrated to
-  the v2 schema (owner-authorized; becomes the new org canonical), so
-  the lll/funlen/cyclop/dupl thresholds now apply; deprecated
-  `gomodguard` disabled in favor of `gomodguard_v2`; fixed the
-  resulting `goconst`, `dupl`, and `lll` findings
+  in `Dockerfile` and `script/bootstrap`); `.golangci.yml` set to the
+  org-standard v2-schema config used across the org's repos
+  (owner-authorized; same file is being landed as canonical via prompts
+  PR #24), with settings under `linters.settings` so the
+  lll/funlen/cyclop/dupl thresholds apply; fixed the resulting
+  `goconst`, `dupl`, and `lll` findings; the informational `gomodguard`
+  deprecation warning under this config is accepted
 - 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints,
   Makefile shims, README Entrypoints section
 - 2026-02-20: iterative DNS resolver implemented; tests made hermetic