feat: release thunderagent program on session end via trajectory_final

[ishandhanani]

What

Adds a deterministic program close for the experimental dynamo.thunderagent_router scheduler. When a pi agent session ends, the provider fires a throwaway max_tokens=1 request carrying nvext.agent_context.trajectory_final=true. The router short-circuits it and releases the program from its table + paused set, so its tokens stop counting against worker utilization. Best-effort — the router's idle/decay reaper is the backstop.

When it fires (multiturn-correct)

On session_shutdown{reason:"quit"} — the single true teardown signal — not on agent_end.

pi emits agent_end once per user prompt. Agents here are multiturn, so the whole session is one trajectory/program (same trajectory_id across every prompt). Hooking agent_end (with a one-shot guard) would:

• close the program after the first turn, then
• leak it for the rest of the session (later prompts re-create an unreleased program), and
• drop the program's router worker/KV affinity mid-conversation.

reason values reload/fork/new/resume keep the same trajectory_id, so the program legitimately persists. print-mode (batch) also emits quit on dispose and awaits the handler, so one-shot runs still close exactly once.

Separate from the subagent session_control close: that frees SGLang KV; this frees scheduler bookkeeping.

Validation

• Unit tests (test/program-close.test.ts): agent_end is not hooked; reload/fork don't close; quit closes exactly once carrying trajectory_final, idempotent.
• End-to-end on GB200 (MiniMax-M2.7-NVFP4 TP2, thunderagent_router):
  • Batch SWE-bench (5 astropy trials): each program released once at trial end; 5 request_end trace spans with trajectory_final: true, output_tokens: 0.
  • Multiturn A/B (3-turn pi RPC session): this change releases the program once at quit, no leak; the old agent_end version released after turn 1 and leaked runs 2-3 (confirmed via a post-exit close probe that reaped the orphaned program).

Related

• Router side: feat(thunderagent): program-close signal (trajectory_final) + resume INFO logging + scheduler docs dynamo#10172 (AgentContext.trajectory_final + end_program short-circuit).