feat(templates): migrate bundled templates to v2-native output (#29)

[diegohb]

Summary

• All bundled templates now render v2-native output: the plugin entry template is Effect-first (Plugin.define, never-throw effect, npm cache advisory), and registration/manifest templates read the v2 plugins key while tolerating legacy keys read-only.
• The plugin-config template renders the v2-native editor, the installer cache root follows the v2 npm cache location, and skill templates drop v1-only frontmatter in favor of v2 skill frontmatter.
• Package manifests (basic and full) declare the v2 server export with latest dependency ranges.
• Review fixes applied to rendered code: the host claim is hedged out of the legacy advisory, BIN instructions are restored, self-labelled read-only config.json mentions are removed, and the Effect v4 catch is gone.

Why

• Issue C6: Migrate templates — generated packages are v2-native and Effect-first #29 asks for the templates to be migrated to v2 so newly scaffolded plugins target the v2 API surface out of the box instead of emitting v1-only frontmatter and keys that rely on compatibility shims.

Testing

• tests/templates.test.ts adds a v1-marker gate over all templates plus gates over the rendered package: typecheck, contract, install, and load-hook, along with a packager-style smoke test that runs the rendered package's own bun test suite.
• Template assertions migrated to v2, covering the npm cache root and v2 skill frontmatter.
• tests/deployment-plan.test.ts updated to match the new template output.