Add dry-run mode for pipeline testing

[renezander030]

What

Add a --dry-run flag that executes pipelines without making real API calls or sending real notifications.

Why

When developing new pipelines, you want to test the flow without actually sending emails, hitting LLM APIs, or pinging Slack. Dry-run mode would log what would happen at each step.

Scope

• Add --dry-run CLI flag
• In dry-run mode: deterministic steps log their action but skip execution, AI steps log the prompt that would be sent but skip the API call, approval steps auto-approve
• Print a summary: "Dry run complete: 3 steps executed, 0 API calls, 0 notifications sent"

Helpful context

• The pipeline runner is in main.go
• This is a good entry point to understand how steps are dispatched