ci(bot): run github actions dependabot updates weekly

[samhamilton]

No JIRA ticket — automated CI / dependency-management change (bot scope).

Why

The github-actions ecosystem moves from monthly to weekly, so action updates — including
security-relevant ones — land within a week instead of up to a month. Every other ecosystem in this
repo keeps its existing cadence.

Change Type

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Testing

• actionlint — clean on every changed workflow (only the known custom martide-runner-set label is reported).
• All changed .yml files parse as YAML.
• Every SHA in the diff was verified against the upstream tag list via GET /repos/{owner}/{repo}/tags.

Code Quality

• Added docs
• Added specs