chore(ci): bump ci-base image node 22 -> 24-alpine

Follow-up to the CI cache work (#635/#637), sequenced separately so the
runtime-version change carries zero cache variables. node:24 is Active
LTS; node:26 is held until it reaches LTS (Oct 2026) since the Current
line risks node-gyp native-module breakage (better-sqlite3, canvas,
sharp, node-pty compile from source on the musl runner).

Only Dockerfile.ci's base changes; ci.yml/publish.yml comments updated
for accuracy. The ci-base image rebuilds automatically on merge (the
Dockerfile.ci path filter in ci-image.yml).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.woodpecker/publish.yml

@@ -2,7 +2,7 @@
 # Runs only on main branch push/tag
 
 variables:
-  # Pre-baked CI base (see .woodpecker/ci-image.yml): node:22-alpine +
+  # Pre-baked CI base (see .woodpecker/ci-image.yml): node:24-alpine +
   # toolchain + warm pnpm store. Kills the second cold install publish pays.
   - &node_image 'git.mosaicstack.dev/mosaicstack/stack/ci-base:latest'
   - &enable_pnpm 'corepack enable'