feat(tui): add server switcher

[opencode-agent]

Summary

• add a client-side registry and <leader>w server picker to the V2 TUI
• validate remote endpoints before switching and persist only normalized URLs
• remount the complete server-scoped provider tree so sessions, routes, events, locations, permissions, and caches cannot leak between servers
• preserve managed-service restart/reconnect behavior only for the startup server

Validation

• cd packages/tui && bun typecheck
• cd packages/tui && bun test (272 passed, 1 skipped)
• cd packages/cli && bun typecheck
• cd packages/cli && bun test test/server-connection.test.ts
• OpenCode Drive: switched local → second V2 server → local using the new keybind and verified each server's distinct model/catalog state

Demo

OpenCode Drive recording: https://slack.com/archives/C0BE69AHCQP/p1784401955303119

Notes

The package-wide CLI suite has one unrelated Host-sensitive failure in concurrent service processes elect one server; the same failure reproduces on the unchanged origin/v2 commit.

Fixes #37667

Requested by: @R44VC0RP (Ryan Vogel via Slack)