Add skill diff command (#46)

[devrimcavusoglu]

Summary

• Add skern skill diff <name> --platform <platform> to compare a registry skill against its installed copy on a platform
• Add skern skill diff <name-a> <name-b> to compare two registry skills by name
• Compare metadata fields (name, description, version, author, tags, allowed-tools) and body content
• Support --json, --scope, and --platform flags

Test plan

• 12 table-driven tests covering both modes (registry-vs-registry, registry-vs-platform)
• Tests for identical skills, different metadata, different body, text output, not-found, missing flags, --platform all rejection, default scope
• make build passes
• make test passes (all packages)
• make lint — 0 issues

Closes #46

🤖 Generated with Claude Code

[devrimcavusoglu]

Code Review

Overall this is a solid, well-tested PR that follows project conventions. All checks pass locally (make test, make lint). A few items to consider:

Issues

1. Missing modified-by comparison — compareSkills covers all Skill fields except Metadata.ModifiedBy. This is the append-only provenance list and a likely source of drift between registry and platform copies. Either add it to the comparison or document its intentional omission.

2. --scope default inconsistency between modes — In two-arg mode, empty --scope searches project-first then user (via resolveSkill). In one-arg (platform) mode, it hardcodes "user". This is pragmatic but subtly inconsistent. The Long description should at minimum note the default.

3. Missing skill.ValidateName call — Other commands (skill install, skill remove) validate names upfront for clearer error messages. This PR skips that step and relies on downstream lookup failures.

4. Long description slightly inaccurate — Says --scope is required for one-arg mode, but it actually defaults to "user". Also --scope applies in two-arg mode too. Suggest:

   With one argument, compares a registry skill against its installed copy on a platform
   (requires --platform; --scope defaults to "user").
   
   With two arguments, compares two registry skills by name
   (--scope filters to a specific scope; omit to search both).
   

Test coverage

12 tests with good coverage of both modes, JSON/text output, and error paths. Nice-to-have additions: --scope project for platform mode, invalid --scope value, two-arg mode with explicit --scope.

None of the above are blocking — approve once addressed or acknowledged.