AB-580 Update Ubuntu runners and GitHub Actions

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Pin hosted Ubuntu jobs to ubuntu-26.04. Upgrade actions/checkout from v6 to v7.

Ticket: AB-580.

Self-review

Reviewed over 1 round with the self-review skill. 0 findings fixed, 0 ignored, 0 second opinions requested, 0 open. The technical and product reviews ran inline. An independent GPT-6 Astra agent evaluated the final committed diff and review record.

Ticket coverage

Status	Criterion	Evidence
✅	Hosted Ubuntu jobs use ubuntu-26.04	committed workflow diff.
✅	Checkout references use the latest major v7	committed workflow diff and release verification.
✅	Other action changes follow the user's per-repository decisions	committed diff and chat approvals.
✅	Only authorized workflow/action changes ship	full branch diff contains only workflow/action YAML.
🟡	Hosted workflow execution	GitHub CI will run after publication. YAML parsing and diff checks passed locally. Commit hooks passed where configured.

Assumptions

• Hosted CI will verify the installed tools and action execution on Ubuntu 26.04.

Risk map — read these first

• .github/workflows/release.yml:27 — Check release tools and Git operations on the new runner.
• .github/workflows/test.yml:9 — Check the existing test jobs on the new runner.

Risk: 🟡 medium

• The committed diff matches the approved runner and action updates.
• Execution on Ubuntu 26.04 still needs validation in GitHub Actions.

🟡 Needs a skim-approve: check the assumptions and validation limits above.