Remove entire TaskState machinery:
- TaskState struct, render(), taskState()/updateTaskState() on History
- inferTaskStateUpdate() — heuristic plan tracking
- Replan gate (replanGate const, roundsWithoutPlan counter)
- Stall detection (stallThreshold, stallRounds, lastAction)
- Plan reinject (planReinjectInterval, periodic re-injection)
- ReadPlanStep plumbing (AgentCfg → Agent → TurnCtx → loop)
None of this worked in practice — the model ignores the nudges,
ReadPlanStep was never wired to a non-nil value, and TaskState
fields were only populated by inferTaskStateUpdate (which depends
on ReadPlanStep being set).
Compaction now uses plain summary text in the cold zone instead
of serialized TaskState JSON.
Also remove:
- Verbosity field (declared, threaded, never read by any backend)
- DisallowTools (config field never wired to session)
- Session.listHandlers, plan, prevPrompt (never read)
- Config.Output, Config.ListHandlers, Config.ReadPlanStep (never set)
-246 net lines
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