[codex] Surface live agent plans in web and CLI v2

[roackb2]

Summary

• Emit plan.updated as a core live conversation activity when the agent successfully calls update_plan.
• Carry that activity through the existing control-plane session event stream instead of adding a separate plan API.
• Render active turn plans in web-v2 and cli-v2, clearing them on run start/session change/run finish.
• Extend shared client session activity handling so both interfaces consume the same API-derived event shape.

Boundary notes

• Plan facts stay owned by core agent planning/update_plan semantics.
• Transport stays in the existing control-plane sessionEvents stream.
• web-v2 and cli-v2 only render the shared activity state; neither imports core/server/old-cli code.

Validation

• ./node_modules/.bin/vitest run src/__tests__/integration/core/run-agent.test.ts src/__tests__/unit/client-shared/session-activity-service.test.ts src/__tests__/unit/cli-v2/control-plane-session-store.test.ts src/__tests__/unit/web-v2/agent-plan-panel.test.tsx
• yarn build
• yarn test