ci: keep the Dart gate, drop the pub.dev publish

ci.yml -- format, analyze --fatal-infos, test -- moves to .hanzo/workflows and
names hanzo-build-linux-amd64. It had asked for ubuntu-latest, which no runner
here advertises, so it queued instead of running.

publish.yml is dropped because pub.dev's automated publishing authenticates the
OIDC token's issuer, and it trusts GitHub Actions as that issuer. Tokens minted
here come from git.hanzo.ai, which pub.dev will not accept, so this is not a
missing credential that can be provisioned -- there is nothing to point the
step at. Publishing this package to pub.dev is a manual `dart pub publish`
until it is fronted by a publisher pub.dev trusts.

.github/workflows/publish.yml

@@ -1,19 +0,0 @@
-name: publish
-
-# Publishes to pub.dev via OIDC when a version tag is pushed.
-# Requires the package to have this repo configured as an automated
-# publisher on pub.dev. Dormant until the first tag.
-on:
-  push:
-    tags: ['v[0-9]+.[0-9]+.[0-9]+']
-
-jobs:
-  publish:
-    permissions:
-      id-token: write
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-      - uses: dart-lang/setup-dart@v1
-      - run: dart pub get
-      - run: dart pub publish --force

.hanzo/workflows/ci.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: hanzo-build-linux-amd64
     steps:
       - uses: actions/checkout@v4
       - uses: dart-lang/setup-dart@v1