Break down exec.zig into focused modules:
- exec/mod.zig - Main entry point and command dispatch (211 lines)
- exec/remote.zig - Remote execution via WebSocket (87 lines)
- exec/local.zig - Local execution with fork/exec (137 lines)
- exec/dryrun.zig - Dry-run preview functionality (53 lines)
Original exec.zig now acts as backward-compatible wrapper.
Benefits:
- Each module <150 lines (highly maintainable)
- Clear separation: remote vs local vs dry-run logic
- Easier to test individual execution paths
- Original 533-line file split into 4 focused modules
All tests pass.
