Record why autocompact cannot be handed to a driver yet

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

TODO.md

@@ -19,10 +19,17 @@ one in place when it turns out to need a decision.
 ## Session settings
 
 - [ ] Autocompact belongs in session settings; empty disables it, which is the
-      default. **Needs a decision before building** — nothing called autocompact
-      exists yet on either side. `PLAN.md` has it only as a planned pi-driver
-      feature (`set_auto_compaction`), and Claude Code runs its own. So this is
-      a new server feature, and the open questions are what the empty-or-not
-      value *is* (a token count? a percentage of the context window?) and which
-      drivers it applies to.
+      default. Iris chose "hand it to the driver" — only where a driver has
+      auto-compaction of its own. **That option was offered on a false premise
+      and is not buildable yet.** It named pi's `set_auto_compaction`, but pi
+      was never built as a driver here: `session/llama.rs` talks to
+      `llama-server`'s OpenAI-compatible endpoint directly, and its `compact()`
+      refuses outright. Claude Code's auto-compaction is the CLI's own and
+      nothing in the stream-json control protocol this app uses configures it.
+      So the setting would be stored, passed to a driver, refused by every one
+      of them, and the field would never appear on any session. What is needed
+      first is either a driver that can take it, or a different rule — the
+      server watching `contextTokens` and running `/compact` itself is the one
+      that would work today, for Claude sessions, and it is the option that was
+      not chosen.