Fix runtime launch when Bun is missing from the Herdr server PATH

[backnotprop]

Summary

• compile the TypeScript entrypoints into one plugin-local executable during installation
• run every action and pane through that relative executable
• keep Bun as an install-time and development dependency only
• add a regression test that runs the compiled action with an empty PATH
• bump the plugin version to 0.3.1

This keeps the fix inside herdr-annotate. It requires no Herdr core changes and no machine-specific Bun path. The generated executable is platform-native, about 61 MB on macOS, and is not committed to Git.

Verification

• bun test: 54 passed
• bun run typecheck
• bun run build:plugin
• linked and parsed successfully with Herdr 0.8.2
• compiled copy-context action completed with Bun absent from PATH

Fixes #5