feat: disable plugin when snip is not installed

[VincentHardouin]

Summary

• Check if snip binary is available in PATH at plugin initialization
• If snip is not found, log a warning and disable the plugin by returning empty hooks
• Inspired by rtk plugin implementation

Fixes #9

Behavior

• snip available: Plugin works normally, prefixing commands
• snip not available: Plugin is disabled, commands pass through unchanged, warning logged

Test plan

• Existing tests continue to pass (plugin behavior unchanged when snip is available)
• Typecheck passes with added dom lib