chore(mise): name the node backend

`node = "22.17.0"` left the backend implicit. It is `core:node` now, so
the install path is explicit. The version is unchanged and checked by
running it rather than by reading the string: `mise -C . exec --
node --version` printed v22.17.0.

Tasks are untouched; all 14 still parse.

No workflow uses mise here, so no CI changes.

mise.toml

@@ -1,5 +1,10 @@
 [tools]
-node = "22.17.0"
+# Backend named so the install path is explicit. Verified by running it:
+# `mise -C . exec -- node --version` after `mise install` printed v22.17.0,
+# matching this value. If the two ever disagree, the install directory is
+# stale rather than this pin being wrong — the same failure surfaced in the
+# forklift repo, fixed by reinstalling and not by moving the version.
+"core:node" = "22.17.0"
 
 [tasks.setup]
 description = "Install Node deps and Playwright browsers"