feat(init): support nitro contracts source overrides

[douglance]

Depends on

• feat: add custom snapshot image baking #34

Supersedes

• feat(init): support nitro contracts source overrides #35

This PR is based on the updated feat/custom-snapshot-bake branch so its diff contains only the Nitro contracts source override work.

Summary

• Allow init and rebuild-based bake workflows to build core rollup contracts from a nitro-contracts branch, tag, or commit.
• Support local nitro-contracts checkouts through a validated BuildKit named context.
• Preserve the pinned defaults and the existing v2.1/v3.2 deployer recipes when no source override is supplied.
• Carry the selected source through detached init runs and the custom-image composite action.
• Use collision-resistant image tags while rebuilding mutable source overrides instead of reusing stale images.
• Document the CLI, environment, and composite-action interfaces.

Validation

• pnpm lint
• pnpm build
• pnpm typecheck
• Full test suite: 230 tests passing
• Docker build checks for the v2.1 and v3.2 deployer Dockerfiles
• Docker build check for the local named-context source path