feat(build): migrate @/* path alias to native package.json subpath imports (#/*)

[cyanheads]

The build is two-step — tsc, then tsc-alias rewrites @/* specifiers in dist/ — because tsconfig paths aliases do not exist at runtime. Node's subpath imports (package.json "imports", #-prefixed) are the native mechanism: tsc under NodeNext resolves them end-to-end, emitted JS needs no rewriting, and declaration files carry working specifiers.

Proposal

Migrate @/* to #/* via a package.json "imports" map; drop tsc-alias from the framework build and from the scaffold (templates carry the same two-step build). ~575 import sites in src/ — mechanical codemod, then the full gate run.

Scope

• package.json "imports" map; remove paths from tsconfig.base.json
• Codemod across src/ + tests/; simplify scripts/build.ts
• templates/: same change for scaffolded servers
• Skills/docs that show the @/ import convention

Out of scope

• Public @cyanheads/mcp-ts-core/* export subpaths (unaffected)

Alternatives considered

• Keep tsc-alias: works today, but it is a build-only rewrite step plus a devDependency in the framework and every scaffolded consumer for something the platform does natively.