feat: add `cprr sync` for worktree .cprr store coordination

## Summary

Add `cprr sync` to coordinate conjectures between main and worktree `.cprr` stores.

## Motivation

Projects using Claude Code worktrees (aria: 6 worktrees, http-axiom: 37 worktrees, qwen3-steering: 4 worktrees) create independent `.cprr/conjectures.json` in each `.claude/worktrees/*/` directory. These diverge from the main store:

- http-axiom main has 41 conjectures; each worktree has 7 (subset)
- qwen3-steering main is corrupted; worktrees have 22 each (superset)

No mechanism exists to merge, diff, or reconcile these.

## Proposed Behavior

```bash
cprr sync # merge worktree stores into main
cprr sync --dry-run # preview merge
cprr sync --worktree DIR # sync specific worktree
cprr list --include-worktrees # aggregate view across all stores
```

## Integration with beads

`bd sync` already exists for bead synchronization. `cprr sync` should follow the same pattern and could share discovery logic for finding worktree directories.

Start by inspecting the existing CLI command structure and the `bd sync` pattern, then trace how `.cprr/conjectures.json` stores are discovered. Define the merge, dry-run, specific-worktree, and aggregate-list behavior; done means `cprr sync` coordinates the stores and the documented command variants work.

Written by the indexing model from the issue text.