feat: support extended fields (assigned_agent, review_gate, priority)

## Summary

Formalize support for extended conjecture fields that are already being used in practice.

## Motivation

The http-axiom project (largest cprr user: 41 conjectures, 37 worktrees) has organically added fields to conjectures that the tool doesn't recognize:

- `assigned_agent` (string) — which agent is responsible for the conjecture
- `review_gate` (string) — blocking question/gate condition before advancing
- `priority` (string/int) — ordering hint for work
- `acceptance_criteria` (string) — what constitutes confirmation
- `falsification` (string) — explicit falsification criteria

These are silently preserved through Go's JSON marshaling but never displayed, queried, or validated.

## Options

**A) Document the extensibility contract** — unknown fields preserved as-is, `cprr show` displays them under "Custom Fields"

**B) Add optional standard fields** to the Conjecture struct for the most useful ones (`assigned_agent`, `priority`, `falsification`)

**C) Generic metadata map** — `cprr add --meta key=value` stored in a `metadata` map field

## Recommendation

Option A is minimum viable. Option B for `falsification` specifically — it aligns with CPRR's Popperian philosophy and the existing `--hypothesis` flag.

Start by reading the Go Conjecture struct and the existing JSON marshaling behavior, then inspect the cprr show and cprr add entry points mentioned in the issue. Compare options A, B, and C against the listed extended fields and the recommendation. Done means the chosen contract is clearly defined and the affected display, storage, querying, or validation behavior is covered.

Written by the indexing model from the issue text.