feat: support personal image sources and custom image builds

[Goultarde]

Summary

• Add personal nihil-images fork and branch management.
• Add interactive per-tool image customization with search and visual selection.
• Support SSH/HTTPS Git remotes and local source cleanup.
• Dispatch selected Docker image variants from the personal branch.
• Store image selections as an enabled_tools allowlist.
• Resolve personal GHCR images using the custom branch tag instead of latest.

Compatibility

• Upstream image sources keep their existing behavior.
• Personal branches without a configured branch fall back to latest.
• Core tools remain mandatory.

Validation

• 117 tests pass.