When scheduler_prompt_injection is enabled and a session_id is
provided on spawn-agent, the daemon builds a PromptAssembly from
the session, serializes it as COLIBRI_SESSION_CONTEXT env var,
and passes COLIBRI_COST_MODE to the spawned agent process.
Config-gated (default: disabled) via COLIBRI_SCHEDULER_PROMPT_INJECTION.
No cache warming — that's PR3b (separate).
Build: pass | Tests: workspace green | Clippy: clean | Fmt: clean
