ci: use Ubuntu for platform-independent release jobs

Run release metadata validation, complete distribution-set validation,
and source archive creation on ubuntu-latest. Keep macOS runners for
building and testing the macOS wheels.

Preserve v0.1.0 history and record that subsequent changes use new commits
instead of amendments unless explicitly authorized for a specific commit.

Validation: actionlint, offline pedantic zizmor, all three release-gate
regression checks, and git diff --check pass.

Author: codex

.github/workflows/artifacts.yml

@@ -82,7 +82,7 @@ jobs:
 
   sdist:
     name: Source distribution
-    runs-on: macos-latest
+    runs-on: ubuntu-latest
     steps:
       - name: Checkout
         uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1

.github/workflows/release.yml

@@ -18,7 +18,7 @@ concurrency:
 jobs:
   metadata:
     name: Validate release metadata
-    runs-on: macos-latest
+    runs-on: ubuntu-latest
     steps:
       - name: Checkout
         uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
@@ -45,7 +45,7 @@ jobs:
   validate:
     name: Validate complete release set
     needs: artifacts
-    runs-on: macos-latest
+    runs-on: ubuntu-latest
     steps:
       - name: Checkout
         uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1

agents.md

@@ -29,6 +29,7 @@
 - Use `MIT OR Apache-2.0`, with Sebastian Thiel as package author and copyright holder. Include both license texts in distributions.
 - Divide work into meaningful, independently buildable and testable commits. Include documentation and relevant regression checks with the behavior they describe.
 - Follow Gitoxide's purposeful conventional commits: use `feat:`/`fix:` for user-visible changes and plain descriptive subjects for maintenance. Use explicit Codex authorship for agent-created commits; package authorship is separate.
+- Preserve tagged and published history. Now that `v0.1.0` exists, create new commits for further changes, including CI fixes; do not amend earlier commits unless the user explicitly authorizes that specific amendment.
 - Keep packaging, type information, examples, contributor guidance, CI, security policy, and artifact-building configuration part of the project.
 - Tests use disposable repositories and isolated Git configuration/environment. Never mutate the developer's checkout, another worktree, or shared Git metadata as a fixture.
 - The implementation sequence and persistent design decisions are in [docs/implementation-plan.md](docs/implementation-plan.md). Update its status as work lands; do not present planned coverage as completed.