Add pipeline listing with DAG visualization across all providers

[faustodavid]

Add comprehensive pipeline listing functionality for GitHub, GitLab, and Azure DevOps with recursive downstream traversal and DAG visualization.

• Add command with for GitLab to traverse downstream pipeline dependencies recursively
• Implement compact DAG output format showing job relationships and status
• Add job normalization logic shared between JobRow and dict builders
• Extend provider contracts (github_builds, gitlab_builds, azdo_builds) with pipeline listing and job retrieval methods
• Add comprehensive formatting utilities for DAG visualization and job tables
• Update CLI parser and handlers to support new pipelines list command
• Add extensive contract tests for all three providers
• Update SKILL.md and documentation with pipeline listing examples and format reference