⚡ Add dry-run mode for pipeline execution

[szmyty]

Title: ⚡ Add dry-run mode for pipeline execution

---

Description

Add a mode that simulates execution without actually running commands.

---

Goals

• Allow users to preview pipeline behavior
• Improve safety and debugging

---

Requirements

• Add flag:

renderflow build --dry-run

• Behavior:
  • log what would happen
  • do NOT execute commands
  • do NOT create files

---

Acceptance Criteria

• Dry-run mode logs intended actions
• No files created
• No external commands executed