Bundle dist outputs and inline SDK runtime

[breznknecht]

Summary

Bundle package outputs so SDK code is in dist artifacts while keeping OpenCode host plugin imports external at runtime.

Changes

• Move @headsdown/sdk to devDependencies and keep @opencode-ai/plugin in dependencies.
• Build bundled outputs for dist/index.js and dist/tui.js with esbuild.
• Add bundled dist/policy.js output for test coverage of policy helpers.
• Run tests against built dist artifacts.
• Document bundling commands and rationale in README.

Compatibility

Exports remain @headsdown/opencode and @headsdown/opencode/tui. Runtime plugin behavior is unchanged.

Validation

• npm run build
• npm test
• npm pack + scratch install check (@headsdown/sdk not installed transitively)
• Runtime import check for @headsdown/opencode and @headsdown/opencode/tui