ci(dependabot): add monthly dependency updates

[0xcucumbersalad]

Summary

Add Dependabot config for monthly dependency updates.

• npm ecosystem (/) — Bun workspaces auto-detected from root package.json; grouped into one PR per run.
• github-actions ecosystem — bumps action versions across .github/workflows/*; grouped.
• Both: monthly schedule, open-pull-requests-limit: 10, commit prefix [chore] matching repo convention.

Testing

Config-only change. YAML validated against Dependabot v2 schema. No code paths affected.

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Summary by cubic

Adds Dependabot to automate monthly dependency updates, replacing manual bumps. Old behavior: no automation; new behavior: grouped monthly PRs for npm (Bun workspaces) and github-actions.

Dependencies

• Runs from the repo root to cover all workspaces.
• Caps open Dependabot PRs at 10.
• Uses [chore] commit message prefix.
• Config-only change with no code or workflow behavior changes.

^{Written for commit 0dcabaf. Summary will update on new commits.}