fix(preview): unpack all three values returned by _auto_compile_prompts

[fmferrari]

Problem

apm preview <script> crashes with:

ValueError: too many values to unpack (expected 2)

This happens for every script definition regardless of content.

Root Cause

_auto_compile_prompts() in src/apm_cli/core/script_runner.py has return type tuple[str, list[str], str] and always returns three values:

return compiled_command, compiled_prompt_files, runtime_content

_execute_script_command() correctly unpacks all three:

compiled_command, compiled_prompt_files, runtime_content = self._auto_compile_prompts(command, params)

But preview() in src/apm_cli/commands/run.py unpacks only two values in both its code paths:

1. The rich-UI try block (line ~142)
2. The except (ImportError, NameError) fallback path (line ~182)

Fix

Add _ to capture the unused runtime_content in both call-sites, matching how _execute_script_command already handles the return value. Preview does not execute the script, so runtime_content is not needed.

Reproduction

Any apm.yml with a scripts: block:

scripts:
  start: "echo hello"

$ apm preview start
Error previewing script: too many values to unpack (expected 2)

Tested against APM 0.19.0.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Updates the preview command to match an updated return signature from script_runner._auto_compile_prompts, preventing tuple-unpacking errors during command preview.

Changes:

• Adjusts tuple unpacking to ignore the new third return value from _auto_compile_prompts.
• Adds an inline comment documenting the updated return shape and why only the first two items are used.