[Feature] Add `pv compare <id1> <id2>` - head-to-head prompt battle

[kumar-ayan]

Currently we can only diff versions of the same prompt.
This lets you compare two completely different prompts against each other.

What to build:

• Run both prompts against the same test cases
• Side-by-side score table: clarity, specificity, eval score, latency
• Declare a winner with reasoning from LLM judge
• Works even if prompts have no test cases (falls back to analysis scores only)

Files to touch: src/commands/ (new compareCmd.ts)