fix(session): keep todo state out of the system prompt

[filipeforattini]

Problem

The legacy session loop (packages/redcode/src/session/prompt.ts) rebuilt the system array every step and included SessionTodo.context(...), which embeds the live task list. Every todowrite therefore changed the system prompt and invalidated the provider prompt-cache prefix for the next request. The goal, by contrast, was already re-rendered into the last user message via SessionReminders.apply, which keeps the system prompt stable.

Fix

• SessionReminders.apply takes an optional todos list and appends SessionTodo.context(todos) to the last user message as a synthetic part, right after the goal reminder. Same text as before, so model behaviour is unchanged.
• The loop reads todos.review once per step and passes the list to the reminders; the system array keeps only the static SessionTodo.guidance. A todo continuation carries its review into the step it starts instead of reading again.
• .changeset/todo-reminder-context.md (@reddb-io/redcode: patch).

Out of scope on purpose: the daily date line and instruction/MCP/skills inputs are left as they are.

Test

New packages/redcode/test/session/prompt.test.ts case: the fake provider replies with a todowrite, then text, then a continuation. The test asserts the role-system messages of requests 2 and 3 are toEqual those of request 1, that the system prompt no longer contains the task-state block, and that request 2's last user message carries Current task state from storage: 0/1 completed with the task text.

• Before the fix the test fails on the system equality with a diff showing No tracked tasks yet replaced by the task-state block; after the fix it passes.
• One existing test that asserted the exact shape of the last user message was updated to the new shape (prompt text first, task-state reminder behind it).
• test/session: 513 pass, 0 fail. bun run typecheck clean.

https://claude.ai/code/session_01KAcoL6wpgVEs2ebUrUGpHS

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