feat: add codex support and optimize runtime transport

[pallyoung]

Summary

• add Codex agent support aligned with the existing Claude session/runtime flow
• unify agent config to the current runtime environment instead of splitting native vs WSL settings
• introduce an agent-client abstraction layer so Claude and Codex share the same upper-layer session/UI logic
• optimize websocket/runtime reload hot paths to reduce duplicate attach work, request fanout, and frontend churn
• collapse several backend DB critical sections on runtime attach/controller guard/disconnect paths
• reduce runtime snapshot session scans during attach assembly

Main areas

• Codex CLI/config/session support aligned with Claude where semantics can match
• runtime reload and workspace transport batching / backpressure improvements
• backend lock acquisition reduction on workspace_runtime_attach, controller release/disconnect, and RPC controller guard paths
• documentation and profiling / regression coverage for Codex compatibility and transport performance

Validation

• node --test tests/runtime-attach.test.ts tests/workspace-artifact-refresh.test.ts
• node --test tests/workspace-runtime-controller.test.ts tests/workspace-view-persistence.test.ts
• pnpm build:web
• cargo test --manifest-path apps/server/Cargo.toml
• CODER_STUDIO_DEV_BACKEND_PORT=43123 CODER_STUDIO_DEV_FRONTEND_PORT=5378 pnpm test:e2e --grep "reload keeps automatic workspace runtime attaches bounded|reload defers session history loading until the drawer is opened|reload replays agent lifecycle history into running and idle browser state|interrupted sessions show an explicit resume entry and reuse the saved claude session id"
• CODER_STUDIO_DEV_BACKEND_PORT=43123 CODER_STUDIO_DEV_FRONTEND_PORT=5378 pnpm test:e2e --grep "reload keeps automatic workspace runtime attaches bounded|observer follows controller and takeover succeeds after timeout|same-device new client takes over immediately after controller disconnects"
• CODER_STUDIO_DEV_BACKEND_PORT=43123 CODER_STUDIO_DEV_FRONTEND_PORT=5378 pnpm test:e2e --grep "reload keeps automatic workspace runtime attaches bounded|interrupted sessions show an explicit resume entry and reuse the saved claude session id"

[pallyoung]

Closing to reopen from develop as requested.