feat(steps): Implement semantic steps framework

[dangazineu]

Create the semantic steps framework and implement core built-in steps for workflow automation.

Description

This issue establishes the semantic steps framework and implements the core built-in steps that provide common workflow functionality.

Built-in Steps to Implement

• tako/checkout@v1 - Repository checkout operations
• tako/update-dependency@v1 - Cross-ecosystem dependency updates
• tako/create-pull-request@v1 - Automated pull request creation
• tako/poll@v1 - Long-running step monitoring

Framework Features

• Step versioning with semantic version support (@v1, @v2)
• Parameter validation and documentation
• Integration with template engine and security model
• Extensible registration system for future steps

Example Usage

steps:
  - uses: tako/checkout@v1
    with:
      ref: main
  - uses: tako/update-dependency@v1
    with:
      name: go-lib
      version: \"1.2.3\"
      ecosystem: go
  - uses: tako/create-pull-request@v1
    with:
      title: \"Update dependency to {{ .inputs.version }}\"
      body: \"Automated dependency update\"

Implementation Details

• Create internal/steps/ package with registration framework
• Implement each built-in step with comprehensive parameter validation
• Add tako steps list command for step discovery
• Integrate steps with execution engine and container runtime
• Create testing framework for built-in steps

Acceptance Criteria

• All built-in steps work correctly with parameter validation
• Step versioning supports semantic version syntax
• tako steps list shows available steps and parameters
• Integration tests validate step functionality
• Framework supports easy addition of new steps

Files to Modify

• internal/steps/registry.go (new)
• internal/steps/checkout.go (new)
• internal/steps/update_dependency.go (new)
• internal/steps/create_pr.go (new)
• internal/steps/poll.go (new)
• cmd/tako/internal/steps.go (new)

Related Issues

• Parent Epic: Execute multi-step workflows #21 - Execute multi-step workflows
• Design: Design(exec): A General-Purpose Workflow Engine for Multi-Repo Automation #98 - Event-driven workflow engine design
• Depends On: feat(engine): Implement containerized execution with security hardening #104 (containerized execution)