feat: surface stale todo state to the model each step

[LeXwDeX]

Why

Todo lists drift silently. todowrite is the only write path (packages/opencode/src/session/todo.ts — delete-all + reinsert, publishes Event.Updated), but once written nothing ever re-surfaces the list to the model: completed work stays pending, stale items linger, and in_progress is never set. The model needs the current list re-injected while uncompleted items exist, or it loses track after a few steps.

Confirmed direction

Original proposal: inject the todo snapshot via PreToolUse additionalContexts, skip on todowrite calls, skip when all done. Exploration refined the seam, keeping the intent:

• PreToolUse additionalContexts are prepended to each tool result (packages/opencode/src/session/tools.ts:188-192), so N parallel tool calls would duplicate the reminder N times in one batch, and pure-reasoning steps (no tool calls) would never see it — exactly where drift is worst.
• Instead, inject once per model step through the existing SessionReminders seam (packages/opencode/src/session/prompt.ts:1682 → packages/opencode/src/session/reminders.ts pattern): a synthetic text part on the last user message, in-memory only (model-visible, never persisted — no transcript bloat).

Skip conditions (user-confirmed, translated to the per-step seam):

• No todos for the session → no injection.
• All todos completed → no injection.
• Freshness guard (replaces the skip-on-todowrite condition): the previous step ended with a successful todowrite in this session → the list was just updated by the model, skip this step.

Scope

• New todo reminder module (sibling of SessionReminders) reading Todo.Service.get(sessionID) at step start.
• Compact single-line-per-item rendering (status + content); only uncompleted items or the full list with statuses — pick the smaller stable rendering.
• Applies to any session holding todos, including child/subagent sessions (todowrite is not in ROOT_ONLY_TOOLS).
• Freshness signal: track the last successful todowrite step per session (via tool execution or Event.Updated subscription).
• Tests: no todos → no part; all completed → no part; pending items → exactly one part per model step with correct statuses; freshness skip after a todowrite step; parallel tool calls yield at most one reminder per model request; reminder parts are not persisted to the transcript; behavior survives compaction.

Acceptance

• With uncompleted todos, every model step (including tool-free steps) carries at most one todo reminder reflecting current statuses.
• After a todowrite step, the immediately following step carries no reminder.
• All-completed and empty lists produce no reminders.
• No duplicate reminders from parallel tool calls.
• Transcript storage unchanged (reminder is model-visible only, not durable).
• Focused session-tool tests and bun typecheck pass from packages/opencode.

Out of scope

• Requiring users to configure hooks.json (this is product-native; users may still layer their own hooks).
• Auto-mutating todo state — the reminder informs; the model still owns updates via todowrite.
• Persistence schema changes or new events.
• DAG node child-session-specific behavior beyond per-session applicability.

Delivery

Separate WHY from #386-#388 (session tooling, not DAG transport). Bind to its own delivery with specgit issue <number> when implementation starts; created via gh directly to avoid clobbering the active end-structured-output delivery record.