docs(core): note CHAT_LLM_CALL_DEADLINE_MS lockstep with turn wall-clock

[yerzhansa]

What

Adds a two-line invariant comment above CHAT_LLM_CALL_DEADLINE_MS in packages/core/src/llm.ts. Comment-only — no behavior change.

export const LLM_CALL_DEADLINE_MS = 300_000;
// Chat calls clip this by the turn's remaining wall-clock (TURN_WALL_CLOCK_MS
// in ./agent/turn-budget.ts), so any value above it never binds — keep <= it.
export const CHAT_LLM_CALL_DEADLINE_MS = 600_000;

Why

Follow-up cleanup surfaced by a /simplify pass over #286. After that change, CHAT_LLM_CALL_DEADLINE_MS (600k) exactly equals TURN_WALL_CLOCK_MS (10 × 60k) and is silently coupled to it: every chat call's effective deadline is min(CHAT_LLM_CALL_DEADLINE_MS, turnBudget.remainingMs()), and remainingMs() is always <= TURN_WALL_CLOCK_MS. So the chat cap only binds while CHAT_LLM_CALL_DEADLINE_MS <= TURN_WALL_CLOCK_MS — raise it above the wall-clock and the bump is inert; drop the wall-clock below it and chat is silently shortened.

That invariant previously lived only in #286's commit message and changeset body (the changeset is consumed/deleted at the next version-bump), leaving no in-code record for a future maintainer editing either constant.

Deliberately a cross-reference comment, not a code coupling: importing TURN_WALL_CLOCK_MS into llm.ts (to derive or assert) would add a wrong-direction dependency edge from LLM dispatch into the agent turn-budget for a rarely-changing constants pair.

Test plan

No behavior change (comment only). Mechanism verified by reading the dispatch site (llm.ts generate() Math.min clip), both chat call sites in coach-agent.ts, and remainingMs() in agent/turn-budget.ts.