ci(deps): automate dependency updates

[alexb-splunk]

Summary

• configure weekly Dependabot updates for uv and GitHub Actions
• keep uv updates lockfile-only, group minor/patch updates, and leave majors as separate PRs
• use a seven-day cooldown for routine version updates; security updates are unaffected
• apply Conventional Commit prefixes so dependency automation does not trigger semantic releases
• update actions/checkout from v5 to v7 and astral-sh/setup-uv from v6 to v8.2.0
• test the supported floor on Python 3.9 and the current release on Python 3.14
• synchronize and validate the committed lockfile before lint, tests, and release checks

Validation

• Dependabot configuration validated against the current Dependabot schema
• GitHub Actions workflows validated against the current workflow schema
• lint workflow commands on Python 3.13
• 12 tests on Python 3.9
• 12 tests on Python 3.14
• wheel and source distribution build

Release impact

This changes repository automation only. The ci(deps) commit is non-releasing under the current semantic-release configuration.

The branches are independent, but merge #1 first so these workflows immediately exercise the refreshed dependency lockfile.