Adds Pi (pi.dev) as an Early Access, disabled-by-default provider on the
orchestrator v2 architecture, speaking Pi's official stdio JSONL RPC mode.

The adapter deliberately spawns the user's own pi install with no --no-*
flags, so extensions, skills, AGENTS.md/SYSTEM.md context, settings.json,
custom models, and auth all load exactly as in the pi TUI. Sessions live in
Pi's own session store and resume via switch_session, so threads stay
interoperable with the TUI in both directions. Extension UI dialogs bridge
into v2 runtime requests; agent_settled (not agent_end) terminalizes turns;
steering uses pi's native steer command.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>