fix: keep terminal mux persistent across navigation

[whoisasx]

Summary

• add a shell-owned terminal mux transport that reconnects across daemon/API base changes while preserving listeners
• refactor visible terminal attachment to switch handles with explicit close/open frames instead of owning the websocket
• harden backend mux data forwarding with an attachment identity guard and cover same-socket switching/cleanup behavior

Fixes #324 terminal transport track.

Tests

• cd frontend && npm test -- terminal-mux useTerminalSession
• cd frontend && npm run typecheck
• cd backend && go test ./internal/terminal ./internal/httpd ./internal/adapters/runtime/zellij

Notes

• Left the separate agent-selection PR track from Initial findings and follow-on iteration to find actual RC: Electron terminal distortion with Codex/zellij #324 untouched.

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