vendor: materialize transitively resolved skills with inherited targets

Summary

Phase 3 of #68 (depends on the recursive resolution sub-issue landing). Wire the recursively-resolved, deduplicated skill set into vendor materialization so transitive skills actually get written out for each configured target.

Scope

• Confirm/implement that a transitively-resolved skill inherits the top-level targets (Claude/Copilot) rather than declaring its own subset, per the design decision.
• Ensure src/vendor/mod.rs's for_target dispatch and the Claude (src/vendor/claude.rs) / Copilot (src/vendor/copilot.rs) adapters materialize each deduplicated skill exactly once, even if reached via multiple paths in the graph.
• End-to-end test in tests/cli.rs: a manifest with one skill whose repo declares a nested ai.json with a further skill, asserting both are materialized for both targets without duplication.

Relevant code

• src/vendor/mod.rs, src/vendor/claude.rs, src/vendor/copilot.rs, tests/cli.rs

Part of #68.

Start with the recursive resolution dependency and read src/vendor/mod.rs, then follow the Claude and Copilot adapters in src/vendor/claude.rs and src/vendor/copilot.rs. Run the relevant end-to-end coverage in tests/cli.rs with a nested ai.json manifest; done means both transitively resolved skills are materialized for Claude and Copilot exactly once.