feat(skills): add destructive command hints to generated skill bundles

[jxom]

Follow-up from #49 / #58.

Summary

Add optional destructive boolean to CommandDefinition. When set, auto-append a confirmation hint ("Confirm with the user before executing this destructive command.") to the command's hint field in generated SKILL.md files.

Changes required

1. src/Cli.ts → CommandDefinition: add destructive?: boolean | undefined
2. src/Skillgen.ts → collectEntries(): read entry.destructive and auto-append hint to cmd.hint
3. src/SyncSkills.ts → collectEntries(): same
4. Tests: verify destructive commands get the confirmation hint, non-destructive commands are unaffected

Context

The reference patch also proposed a mutates flag with a --dry-run hint, but that assumes the command supports --dry-run which isn't always true. Starting with just destructive is safer.