docs(turn-loop): document hyperhive.autoCompact nix option

The compaction section already documented HIVE_COMPACT_WATERMARK_TOKENS
but didn't mention the per-agent nix option (hyperhive.autoCompact,
default true) that maps to it.

Add a short note after the watermark paragraph explaining that operators
can set autoCompact = false in agent.nix to disable proactive compaction,
and note the reactive path still applies. Useful for sonnet/opus agents
where the 75% heuristic fires before the session is actually full.

docs/turn-loop.md

@@ -199,6 +199,17 @@ or `/compact` is surfaced as a `Note` but never fails the turn that
 already succeeded. The operator can also force a compaction any time
 via `/api/compact`.
 
+To disable proactive compaction for a specific agent, use the nix option:
+
+```nix
+hyperhive.autoCompact = false;  # default true
+```
+
+Setting `autoCompact = false` is equivalent to `HIVE_COMPACT_WATERMARK_TOKENS=0`.
+Useful for agents running large-context models (sonnet/opus) where the 75%
+heuristic fires before the session is actually full — the reactive path
+(compact-on-overflow when the session hits the hard limit) still applies.
+
 - **Auto session-reset** — a third path that fires when both
   conditions hold: context is ≥ a watermark (`HIVE_AUTO_RESET_WATERMARK_TOKENS`,
   default **50% of `context_window_tokens(model)`**) AND the time since