chore: release 0.13.0 (#60)

## Summary

Bump version `0.12.0` → `0.13.0`.

Features since 0.12.0:
- Agent-agnostic support — Claude Code + Codex CLI (#52)
- `crab agent sync` for cross-agent config migration (#55)
- `crab env snapshot/restore` for machine migration (#57)
- Bats unit test framework + CI (#52)
- `crab court` fresh-start prompt fix (#54)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: MrFlounder

src/crabcode

@@ -27,7 +27,7 @@
 
 set -e
 
-VERSION="0.12.0" # x-release-please-version
+VERSION="0.13.0" # x-release-please-version
 CONFIG_DIR="$HOME/.crabcode"
 CONFIG_FILE="$CONFIG_DIR/config.yaml"
 WIP_BASE="$CONFIG_DIR/wip"