Document the cross-feature dependency graph

[tomgrv]

Problem

Each feature declares dependsOn/installsAfter relationships inside its own devcontainer-feature.json, but there's no single place to see the resulting graph across all 9 features. Today, common-utils is a foundation depended on (directly or transitively) by most other features:

common-utils (foundation)
  └─ gitutils, gitversion, act, pecl, gateway, minikube
       └─ githooks (depends on gitutils)
       └─ larasets (depends on common-utils, docker-in-docker)

Without a visible graph, a contributor changing common-utils (or any shared dependency) has to manually trace every feature's manifest to understand the blast radius — which is part of why version-sync mistakes happen (see the separate version-sync issue).

Proposal

• Add a generated dependency graph (e.g. a small script that parses every src/*/devcontainer-feature.json dependsOn/installsAfter and emits a Mermaid diagram or DOT graph) checked into the repo or rendered in the README.
• Regenerate it in CI (or as a pre-commit step) so it can't silently go stale as features are added/changed.

Where this lives / sync note

This is a root-level documentation/tooling addition (reads from all src/*/devcontainer-feature.json but doesn't modify feature source), so it doesn't need special handling for the split-repo sync — it's monorepo-only tooling.

Acceptance criteria

• A dependency graph (diagram or structured doc) covering all current features exists in the repo.
• It's regenerated automatically (CI or pre-commit) rather than maintained by hand.