Builds on #4132 · Closes #4105

Description

Relationship to #4132

#4132 fixed the host-advance ID collision (static "host-advance" → unique host-advance-{seq}-{idx}), ensuring multiple complete_step calls within a single turn correctly update the TodoPanel. This PR addresses the remaining half of #4105: the panel still reverted after tab switch or restart because the synthetic events were never persisted to the session file.

Problem

Host-advance synthetic todo_write events (emitTodoState) update the frontend TodoPanel in real time but are emitted to the event sink, not written as provider.Message to the session file. When the session is reloaded (restart, tab switch, reconnect) the frontend rebuilds its transcript from the session file and only sees the last explicit model todo_write, missing every intermediate host-advance.

Reproduction (after #4132):

1. Model todo_write: [A:in_progress, B:pending, C:pending]
2. complete_step A → panel shows 1/3 ✓
3. complete_step B → panel shows 2/3 ✓
4. Restart or switch tabs and back → panel reverts to 1/3 ✗

Fix

Two complementary mechanisms:

1. HistoryWithCanonicalTodos — appends a synthetic assistant message carrying the canonical todo state at the end of the history response. Called from HistoryForTab (desktop) and GET /history (HTTP).

2. EmitCanonicalTodoState — emits the canonical state as a live event through the sink, so the frontend receives it even when tab state is served from the in-memory cache without a full history reload.

Preserved (unchanged)

• All guard mechanisms: verifyTodoCompletionTransitions (feat(agent): guard todo completion transitions #2576), final-answer gate (fix(agent): block final answers with incomplete todos #2920), advanceCanonicalTodo / recordTodoState (fix(agent): close complete_step cross-turn evidence + loop gaps #4014)
• Frontend TodoPanel / transcript rendering logic
• Session save/load format

Files changed