fix(prompt): keep plan/build reminders stable

[namnguyen0503]

Issue for this PR

Closes #26749

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Fixes Plan/Build <system-reminder> prompt-history drift.

Previously, a historical user message could be serialized once as Continue plus the Plan -> Build reminder, then later serialize as only Continue. That changes already-seen model input and breaks prefix cache stability for llama.cpp, vLLM, LM Studio, and other prefix-caching backends.

This PR anchors the reminder to a stable turn boundary and avoids mutating historical message parts during prompt construction.

How did you verify your code works?

Ran:

• bun test test/session/prompt.test.ts -t "keeps plan/build system reminders anchored to the original user turn across later serializations"
• bun test test/session/prompt.test.ts -t "static loop consumes queued replies across turns"
• bun test test/session/prompt.test.ts
• bun turbo typecheck

All passed.

Also ran a source-checkout smoke test with the repo fake OpenAI-compatible TestLLMServer. It verified that the historical Continue + <system-reminder> message stayed byte-stable across a later Build turn.

Screenshots / recordings

Not applicable. This is a prompt serialization bug fix covered by tests.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on the search results, I found two related PRs that might be addressing similar issues:

1. PR fix(session): persist plan mode reminders #26743 - fix(session): persist plan mode reminders

   • fix(session): persist plan mode reminders #26743
   • This PR also deals with plan mode reminders and was referenced by the current PR (it closes Plan mode reminders are not persisted in history, breaking prefix cache stability #26749, which is related to plan reminders)

2. PR fix(session): drop stale plan reminders #24343 - fix(session): drop stale plan reminders

   • fix(session): drop stale plan reminders #24343
   • This is another earlier PR that addressed stale plan reminders, which could be related to the stability issues being fixed

Why they might be related:

• All three PRs focus on plan/build reminder handling in the session/prompt system
• PR fix(session): persist plan mode reminders #26743 is particularly relevant since the current PR closes issue Plan mode reminders are not persisted in history, breaking prefix cache stability #26749, which appears to be part of the same prompt stability problem
• These PRs appear to be iterating on the same aspect of the codebase (plan reminder persistence and stability)

You may want to check if PR #26743 has already been merged or if there's any overlap in the fixes being applied.