Migrate from Dependabot to Renovate for dependency updates

[benhigham]

Problem

Dependabot requires explicit directories entries for each composite action path to discover uses: references outside .github/workflows/. This was the root cause of the version skew fixed in #32 — claude-invoke/action.yml was missed because Dependabot only scanned /.

Renovate auto-discovers all files containing GitHub Actions uses: references without per-directory configuration, making it more robust for repos with composite actions.

Suggested scope

• Install the Renovate GitHub App (or configure self-hosted)
• Create renovate.json covering all 7 ecosystems currently in dependabot.yml: github-actions, npm, docker, terraform, devcontainers, gitsubmodule, gomod
• Preserve existing grouping, labelling, and commit-message conventions
• Remove .github/dependabot.yml once Renovate is confirmed working
• Update AGENTS.md to document the new dependency management tool

Context

• Current Dependabot config: .github/dependabot.yml
• Fixed in Align claude-code-action version between claude-invoke and downstream workflows #32: added directories entries as a stopgap for composite action scanning
• Renovate advantages: auto-discovery, richer PR descriptions, more flexible grouping and scheduling