fix(session): fire system.transform before messages.transform

[ualtinok]

Issue for this PR

Closes #19960

Type of change

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

What does this PR do?

Reorders the plugin hook execution so experimental.chat.system.transform fires before experimental.chat.messages.transform. Previously, messages.transform ran first in prompt.ts while system.transform ran later inside llm.ts — plugins couldn't inspect the final system prompt when mutating messages.

The system prompt assembly (agent prompt + env + skills + instructions + user.system) and the system.transform hook + header caching logic were extracted from llm.ts into shared helpers (LLM.buildSystem, LLM.transformSystem). These are now called in prompt.ts before messages.transform, and llm.ts receives the already-finalized system prompt.

Also adds sessionID and model to the messages.transform input (previously empty {}), so plugins can correlate transformations across hooks.

agent.ts's independent system.transform call (for agent generation) is unchanged — different code path.

How did you verify your code works?

• Typecheck passes clean
• Full test suite: 1625 pass, 0 fail, 8 skip (identical to dev baseline)
• Verified LLM.buildSystem produces the same system prompt shape as the old inline assembly in llm.ts
• Verified LLM.transformSystem preserves the header caching rejoin behavior

Checklist

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