feat: add skip_install input for pre-installed CLI runners

[qwen-code-dev-bot]

What this PR does

Adds a skip_install input (default: false) that skips the npm/pnpm installation step when the Qwen Code CLI is already pre-installed on the runner.

Why it's needed

Self-hosted ECS runners (like those used in qwen-code's Autofix/triage workflows) maintain a persistent qwen CLI installation. The current action always runs npm install --global on every invocation, which is redundant and adds ~30s of overhead per run.

This pattern was already adopted in qwen-code's own workflows:

• PR #4866 (qwen-code) proposed using pre-installed qwen in ECS workflows
• PR #6207 (qwen-code) adopted a similar approach in Autofix

But qwen-code-action itself has no way to skip the install step — workflows that use the action on pre-installed runners still pay the install cost.

Changes

1. New input: skip_install (string, default 'false')
2. Install pnpm step: skipped when skip_install == 'true'
3. Install Qwen Code step: skipped when skip_install == 'true'
4. New step: "Verify pre-installed Qwen Code" — runs when skip_install == 'true', checks that qwen is in PATH and fails with a clear ::error if not

Usage

- uses: QwenLM/qwen-code-action@v1
  with:
    skip_install: 'true'
    openai_api_key: ${{ secrets.OPENAI_API_KEY }}
    prompt: '...'

Risk & Scope

• Default is false — existing workflows are unaffected.
• The verify step fails fast with a clear error message if qwen is not found, so misconfiguration is caught immediately.
• No changes to the Run Qwen Code step or any other behavior.