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

Replace the commit-pinned golangci-lint install refs in Dockerfile and
script/bootstrap with the v2.12.2 version pin, and replace .golangci.yml
with the canonical v2-schema config (lint settings now live under
linters.settings, so the lll/funlen/cyclop/dupl thresholds are actually
applied).

Fix all findings surfaced by the newly-applied config:

- goconst: introduce 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

TODO.md

@@ -25,6 +25,10 @@ confirm make check still passes.
 
 # Completed Steps
 
+- 2026-08-07: golangci-lint bumped to v2.12.2 (pins in `Dockerfile` and
+  `script/bootstrap`), `.golangci.yml` replaced with the canonical v2
+  config (settings moved under `linters.settings` so thresholds now
+  apply); fixed all resulting `goconst`, `dupl`, and `lll` findings
 - 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