fix: add pre_start_script support to claude-code module for dependency ordering

[jasonwbarnett]

Problem

When using multiple startup modules in a Coder workspace template (e.g., git-clone and claude-code), there's a race condition where both modules' startup scripts execute in parallel. This causes issues when one module depends on another.

Specific Case: git-clone → claude-code

The claude-code module initializes with a workdir that should be populated by the git-clone module. However:

1. All workspace agent startup scripts execute in parallel via Go's errgroup.Group
2. The git-clone module's post_clone_script (which creates the workdir) runs sequentially AFTER the clone completes
3. The claude-code module's startup scripts run concurrently and may fail if they try to access the workdir before git-clone completes

Root Cause

The Coder agent executes all startup scripts with concurrently. There's no built-in mechanism to enforce sequential execution between different coder_script resources or modules.

Solution

Add a variable to the claude-code module that allows users to run custom shell commands before the Claude Code startup script executes. This enables dependency management:

1. claude-code's pre_start_script can wait for markers/conditions from other modules
2. Once preconditions are met, the main start script proceeds
3. This provides a clean way to handle module dependencies without architectural changes

Use Case

This pattern is already used in other parts of the Coder registry (pre_install_script, post_install_script) and would provide consistency.

[jasonwbarnett]

Thanks @DevelopmentCats I will just use the pre_install_script in claude-code module.