planning: replace the mandatory pre-turn gate with an agent-invoked plan tool ...

- Phase 2 (critique) retired; observe/MODE and the top-level DIRECT shortcut
  removed (sub-agents keep DIRECT for trivial subtasks)
- replan merged into plan: `reason` (+ updated_goal) packs the re-plan
  context, without a reason it is fresh planning
- both phases call the LLM with think=False — cloud reasoning models leak
  their chain-of-thought into structured output on non-streaming calls —
  and strip the gemma "thought<channel|>" content artifact
- Phase 1 conversation windowed to ~20k chars, newest-first with the
  original task pinned (28.7k-token prompts came back as 1-token output)
- confirmation by COMPLEXITY: the tool result tells the agent to present
  complex plans and wait; sub-agents keep the execute-now injection
- PlanningStatus/PlanReady reach the UI mid-turn via current_event_sink
  (ctx.event_sink is None in the agent loop — events silently never
  reached the WS before)
- agent gate, casual-message detector and adaptive re-plan nudges removed;
  anti_stall keeps only stall detection