Extends GatewayEvent union with structured_part variants, adds matching
HistoryPart types, wires SSE emission in runtime handleChat, and maps
agent-core content parts to history in session-state.
Complete the remaining pi-to-companion rename across companion-os, web, vm-orchestrator, docker, and archived fixtures.
Verification:
- semantic rg sweeps for Pi/piConfig/getPi/.pi runtime references
- npm run check in apps/companion-os (fails in this worktree: biome not found)
Co-authored-by: Codex <noreply@openai.com>
