Lazy-load the heavy runtime behind a lightweight extension entrypoint

[leo-cheron]

Problem

Installing pi-agent-browser-native as a normal Pi/OMP extension eagerly imports its large runtime graph during every session startup, even when no browser work is requested.

Controlled OMP 18.0.3 RPC benchmark (--no-session --no-skills --no-rules, isolated profile/cwd, 5 warmups, 30 samples):

Configuration	Median
No extensions	1.28 s
pi-agent-browser-native@0.5.0 loaded directly	1.69 s
Same 0.5.0 entrypoint behind a dynamic-import façade	1.23 s

The eager entrypoint adds about 410 ms. This is import/initialization cost, not browser launch; the package already avoids spawning browser subprocesses during registration.

Proposal

Split the extension into:

1. A lightweight entrypoint that registers the public tool schema and essential lifecycle hooks.
2. A heavy runtime module loaded once, via a shared dynamic-import promise, when agent_browser is first executed or browser-specific prompt handling is required.

The façade should preserve current lifecycle behavior by retaining/replaying the active session context when the runtime initializes. Please account for session_start, session_tree, session_shutdown, prompt guidance, conditional agent_browser_web_search registration, and cleanup behavior.