Add `if` field to tasks and commands that allows conditional execution
based on a shell command exit code. When the condition returns non-zero,
the task/command is silently skipped (with verbose logging).

- Task-level: skip entire task if condition fails
- Command-level: skip individual command if condition fails
- Supports template variables in conditions
- Works with for loops (condition evaluated per iteration)
- Works with deferred commands

Add comprehensive test cases for:
- Task calls in cmds with if conditions
- Go template functions (eq, ne, and, or)
- Direct template variable evaluation
- CLI variable overrides
- Task-level if with templates

Add comprehensive golden fixture tests for if feature:
- Command-level if (true/false)
- Task-level if (true/false)
- Task calls with if conditions
- Go template functions (eq, ne, and, or)
- Direct template variable evaluation
- CLI variable overrides
- For loops with if conditions

Co-authored-by: Andrey Nering <andreynering@users.noreply.github.com>