feat(exec): Implement comprehensive dry-run mode

[dangazineu]

Implement comprehensive dry-run functionality for multi-repository event-driven workflows.

Description

This issue implements advanced dry-run capabilities that provide detailed insights into what a workflow execution would do without actually performing any actions.

Dry-Run Features

• Execution plan visualization across repository boundaries
• Event simulation with subscription evaluation
• Template variable resolution without execution
• Resource requirement analysis and validation
• Dependency graph visualization

CLI Usage

# Basic dry-run
tako exec --dry-run release

# Multi-repository dry-run with event simulation
tako exec --dry-run release --repo=my-org/go-lib --inputs.version-bump=minor

# Subscription debugging
tako exec --dry-run release --debug-subscriptions

# Template resolution preview
tako exec --dry-run integration_test --show-templates

Example Output

Execution Plan for: release (my-org/go-lib)
├── Step: build_library
│   ├── Command: ./scripts/build.sh --bump minor
│   ├── Templates: {{ .inputs.version-bump }} → minor
│   └── Events: library_built (schema v1.0.0)
├── Step: fan_out_to_dependents
│   ├── Event: library_built
│   ├── Subscribers: my-org/app-one, my-org/app-two
│   └── Wait: true (timeout: 2h)
└── Expected Execution Tree:
    ├── my-org/go-lib [parent]
    ├── my-org/app-one [subscriber: library_built]
    └── my-org/app-two [subscriber: library_built]

Implementation Details

• Add execution plan generation for multi-repository scenarios
• Implement event simulation for debugging subscription filters
• Support template resolution display without side effects
• Create dependency graph visualization
• Add resource requirement analysis and validation

Acceptance Criteria

• Dry-run shows execution plan across all affected repositories
• Event simulation evaluates subscription filters correctly
• Template variables resolved and displayed without execution
• Dependency graph visualization aids debugging
• No side effects occur during dry-run mode

Files to Modify

• internal/engine/dryrun.go (new)
• internal/engine/simulation.go (new)
• cmd/tako/internal/exec.go (extend)

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 tako/fan-out@v1 semantic step #105-feat(engine): Implement subscription-based workflow triggering #106 (event-driven implementation)