fix(opencode): collapse system prompt fragments into a single system message - #2

fix/system-prompt-normalization

Title:

fix(opencode): collapse system prompt fragments into a single system message

Body:

## Summary

This normalizes the final system prompt into a single system message before provider request construction.

## Problem

`LLM.stream()` builds a `system: string[]` from:
- provider/agent prompt
- `input.system`
- `user.system`
- plugin mutations via `experimental.chat.system.transform`

That array was then mapped to multiple separate `role: "system"` messages for most providers.

Some strict backends/templates are sensitive to this representation, and multiple system messages can cause prompt
handling failures.

## Fix

Collapse the final `system` fragments into a single newline-joined system string before building provider messages.

This keeps all system content while avoiding multiple system-message entries in the final request.

## Why this is safe

- No system content is removed
- Only the representation changes
- Plugin modifications still apply before normalization
- Reduces provider/template incompatibility for strict backends

## Tests

Added unit tests for:
- collapsing multiple fragments into one system prompt
- dropping empty fragments
- returning `[]` when all fragments are empty

## Verification

Validated locally with:
- `bun typecheck`
- `bun test test/session/llm-system.test.ts`

Related to:
- anomalyco/opencode#15059