Support full cross-schema resolution of refs

[mindsocket]

Issue #15 did not address resolution of cross-file references at multiple levels, although this was noted in schema.ts: "Full cross-schema traversal will be addressed when template-sync is updated in #15.". It currently only does this one level deep.

[mindsocket]

This issue blocks several other issues:\n- #17 (Schema composability): Cannot implement shared building blocks and team-composed schemas without full cross-schema resolution\n- #31 (General relationships): Relationship constructs may need to reference schemas across different files\n- #28 (Schema metadata validation): Needs to resolve references to validate metadata correctly\n\nPriority should be given to resolving dependencies before tackling these higher-level features.