fix: merge adaptive guidance into existing system prompt

[zoomdbz]

Summary

Some strict chat templates require the system prompt to appear only at the very beginning of the conversation. The plugin's experimental.chat.system.transform hook appended its adaptive-thinking guidance with system.push(...), which adds a second system message. Against those templates that fails to render before inference even starts. For example, a local llama.cpp provider raises Jinja Exception: System message must be at the beginning.

Fix

Merge the adaptive-thinking guidance into the existing first system entry rather than pushing a new one. When no system prompt exists yet, the guidance is still injected on its own, so behavior is unchanged for templates that allow it.

const existingSystemPrompt = system[0];
if (existingSystemPrompt === undefined) {
  system.push(adaptivePrompt);
} else {
  system[0] = `${existingSystemPrompt}\n\n${adaptivePrompt}`;
}

This is provider- and model-agnostic: it is compatibility with strict chat templates in general, not a workaround for one model.

Automatic reasoning-effort switching (set_reasoning_effort, temporary vs persistent variants, session.idle reset, current-variant reporting) is unchanged.

Tests

Added coverage in src/index.test.ts (19 passing):

• existing system prompt: guidance appended to the first entry, exactly one entry remains, original content preserved, guidance present
• empty system prompt: guidance injected, exactly one entry
• current effort, valid variants, and configured tool name still included

Validation

pnpm format:check, pnpm lint, pnpm typecheck, pnpm test (19/19), pnpm build all pass.