chore: reduce dependabot frequency and group updates

[randomm]

Task Description

Reduce dependabot CI costs by switching to monthly schedule and grouping dependency updates.

Current state:

• Weekly schedule for both cargo and github-actions ecosystems
• No grouping — each dep gets its own PR and CI run
• ~10 PRs/month, each triggering CI on 3 platforms

Changes:

1. Change schedule interval from weekly to monthly for both ecosystems
2. Add grouping rules to batch minor/patch updates into single PRs
3. Keep major version updates as separate PRs (breaking changes need individual review)

Quality Gates (Non-Negotiable)

• Linting: YAML is valid
• Local Verification: Config syntax verified

Acceptance Criteria

• .github/dependabot.yml updated with monthly schedule
• Grouping rules added for both cargo and github-actions
• Major updates remain ungrouped