Stage 2 lands the agent loop on top of the Stage 1 scaffold: session
state with per-session cancellation, a system-prompt builder honouring
HELEXA_ACP_SYSTEM_PROMPT_PATH / system_prompt_path TOML, and handlers
for initialize / session/new / session/prompt / session/cancel that
stream provider output back as session/update notifications. Verified
end-to-end against cortex from Zed.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
