fix: make Claude skill install opt-in

[ekkoitac]

Fixes #483

What changed:

• Stop adding scripts.postinstall to the published dist package.json when src/ax/scripts exists.
• Keep the explicit ax CLI entry, copied skills, and bundled scripts/postinstall.mjs available for opt-in use through npx @ax-llm/ax setup-claude and npx @ax-llm/ax remove-claude.
• Add a postbuild regression test for the case where package scripts are bundled.

Validation:

• npm ci completed.
• npx vitest run scripts/postbuild.test.js passed before the change with 3 tests.
• npx vitest run scripts/postbuild.test.js passed after the change with 4 tests.
• npx biome format scripts/postbuild.js scripts/postbuild.test.js reported no fixes.
• git diff --check passed.
• Manual postbuild inspection of src/ax/dist/package.json: scripts is absent, bin.ax still points to ./cli/index.mjs, dist/scripts/postinstall.mjs is still copied, and 15 skill markdown files are still copied under dist/skills.

Not run:

• Full npm run test; this patch only changes postbuild package metadata and the matching regression test.