feat(plugin): expose platform-safe external URL opener

Problem

V2 TUI plugins that open documentation, logs, pull requests, or other external URLs currently need to spawn platform-specific commands such as macOS open. This leaks host/platform concerns into plugins and does not work consistently on Linux or Windows.

Proposed Direction

Expose a small host-owned plugin API, for example:

context.app.openExternal(url)

The host should validate the URL and dispatch through the platform-appropriate mechanism.

Acceptance Criteria

• Available to V2 TUI plugins through Plugin.Context.
• Supports at least http: and https: URLs.
• Works on macOS, Linux, and Windows through host-owned behavior.
• Rejects unsupported or malformed schemes with a useful error.
• Has focused platform/validation tests.

Scope

This is a generic host capability. Plugins remain responsible for deciding which URL to open and when.