#209 fix: mount force-input hint through TUI renderer

Summary

• Return the footer contribution as JSX so OpenTUI mounts it inside the renderer.
• Register the force-submit keymap layer once during plugin setup.
• Update local testing instructions to load the built package directories.

Testing

• bun run test (force-input package)
• bun run typecheck (force-input package)
• bun run build (force-input package)
• bun run check (force-input package and workspace)

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Summary by cubic

Fixes the force-input plugin mounting its footer hint through OpenTUI's renderer instead of injecting it into the UI manually.

• The force-submit keymap layer now registers once during plugin setup instead of on every footer render.
• The hint contribution returns JSX so OpenTUI mounts it via its renderer; the build entry moves to src/index.tsx.
• Local testing instructions now load built package directories and describe a manual verification flow.