fix: stabilize workspace transport and session persistence

[pallyoung]

Summary

• move hot workspace mutations onto websocket-first transport with HTTP fallback, including session updates, agent resize, terminal resize, and controller heartbeat
• debounce session activity persistence and workspace view sync so drag/input heavy flows stop spamming RPC calls while keeping UI repaint and terminal fit behavior stable
• harden terminal/session stream restore paths to reduce garbled UTF-8 output, preserve snapshot merges, and add coverage for runtime attach, layout persistence, and transport fallbacks

Test Plan

• node --test tests/agent-service.test.ts tests/workspace-route-runtime.test.ts tests/workspace-runtime-controller.test.ts tests/ws-rpc-fallback.test.ts tests/workspace-ready-runtime.test.ts tests/session-service.test.ts tests/session-actions.test.ts tests/session-title-persistence.test.ts tests/session-status-display.test.ts
• pnpm build:web
• cargo check --manifest-path apps/server/Cargo.toml

[pallyoung]

Closing this PR and keeping the changes directly on develop.