Fixes #4170

Also addresses #4169 (UI shows only 1 completion instead of all).

Problem

When a plan is approved, the controller emits a plan-seed todo_write event so the frontend task panel populates immediately. However, the controller did not initialize the agent's todoState — that internal state is only set when the model calls todo_write itself.

Since the plan-seed already displays the list, the model often skips calling todo_write and goes straight to complete_step. When complete_step runs, advanceCanonicalTodo checks len(todoState) == 0 and returns early — no synthetic todo_write update event is emitted, so the frontend counter stays at 0/x.

Root Cause

seedPlanTodos in controller.go emits UI events but does not initialize the agent's canonical todo state. The agent and the frontend were out of sync: the frontend had the list, but the agent did not.

Fix

1. Agent.SeedTodoState() — new public method that initializes todoState when empty (no-op if already set, so a model's own todo_write is never overwritten)
2. Controller.seedAgentTodoState() — parses the plan-seed args into []evidence.TodoItem and calls SeedTodoState
3. Called from seedPlanTodos after emitting the plan-seed events

Now when complete_step runs after a plan-seed:

• todoState is populated → advanceCanonicalTodo finds the matching item
• The item is marked completed + next item promoted → synthetic todo_write emitted
• Frontend updates the counter: 1/5 → 2/5 → ... → 5/5

Tests

• TestSeedTodoState — seeds empty agent
• TestSeedTodoStateNoOverwrite — no-op when agent already has state
• TestSeedTodoStateAllowsAdvanceAfterSeed — proves the full flow: seed → advance → counter updates
• TestSeedPlanTodosSeedsAgentState — controller integration: plan-seed also seeds agent
• TestSeedPlanTodosEmptyPlanNoOp — empty plan is a no-op

Files Changed

• internal/agent/agent.go — SeedTodoState() method
• internal/agent/canonical_todo_test.go — 3 new agent tests
• internal/control/controller.go — seedAgentTodoState() helper + call from seedPlanTodos
• internal/control/plan_seed_test.go — 2 new controller tests