feat(cli): add offline result compare command

[googs1025]

Background

Now skill-up can run evals, generate reports, and create baseline benchmark data. But I think it still misses one offline command to compare two finished runs.

When we change a Skill, review a PR, or upgrade model, we often want to know:

• which cases are fixed
• which cases are regressed
• how pass rate changes
• how tokens and duration change
• whether engine or model is different
• whether this can be used as a CI gate

Proposal

Add a new command:

skill-up compare <old-result.json> <new-result.json>

It should compare two existing result/report JSON files. It should not run eval again, and should not call any agent or judge.

Scope

The command can:

• read JSON files compatible with current report.Input/result.json format
• use primary case result semantics, so benchmark without_skill entries are not counted twice
• show run level delta:
  • pass rate
  • case counts
  • total/input/output tokens
  • duration
• show case level transition:
  • fixed
  • regressed
  • unchanged
  • added
  • removed
• show metadata difference, like skill name, engine, model, schema version and time
• support default text output
• support --format json for CI or other tools
• support optional gates, for example fail on regression or too much token increase

Non-goals

• Do not rerun evals
• Do not call agent or judge
• Do not add dashboard/cloud/snapshot feature in this issue
• First version does not need markdown output

Acceptance criteria

• skill-up compare old.json new.json prints a readable summary
• --format json prints stable machine readable output
• benchmark result is not double counted
• fixed/regressed/added/removed cases are classified correctly
• configured gate failure returns non-zero exit code
• invalid input gives clear error message