Fix TypeScript runtime .js import resolution

[GYF0311]

Summary

• Resolve TypeScript NodeNext / ESM runtime .js, .jsx, .mjs, and .cjs import specifiers to source files when the exact runtime file does not exist.
• Preserve exact runtime file matches first, so real .js files still win.
• Add .mts / .cts source probes and a regression test for ./foo.js resolving to foo.ts.

Root Cause

The resolver previously appended source probes to the full import specifier. For example, ./commands/source.js would probe paths like commands/source.js.ts, miss the existing commands/source.ts, and drop the deterministic import edge.

Validation

• pnpm --filter @understand-anything/core build
• pnpm --filter @understand-anything/skill build
• pnpm test — 16 test files, 201 tests passing
• Verified a local TypeScript ESM fixture now produces deterministic internal import edges where it previously produced none.