docs(claude-code): document pre_install_script for module dependency ordering

[jasonwbarnett]

Summary

Clarifies that the existing pre_install_script variable can be used to handle dependencies between modules during workspace startup.

Problem

When using multiple startup modules (e.g., git-clone and claude-code), there's a race condition where scripts execute in parallel. Module dependencies need to be managed, such as ensuring git-clone completes before Claude Code tries to access a workdir.

Solution

The existing pre_install_script variable already provides this capability. Updated documentation to clarify this use case.

Example

module "claude-code" {
  source = "registry.coder.com/coder/claude-code/coder"
  
  workdir = "/path/to/repo"
  
  # Wait for git-clone to complete before starting
  pre_install_script = <<-EOT
    #!/bin/bash
    set -e
    while [ ! -f /tmp/.git-clone-complete ]; do
      sleep 1
    done
  EOT
}

Resolves issue #609.