test: Add E2E integration tests for hook slot verification (mi-3zc)

Add comprehensive integration tests covering hook slot operations:
- BasicHook: Verify bead can be hooked to an agent
- Singleton: Document that bd allows multiple hooks (gt enforces singleton)
- Unhook: Verify hook removal via status change
- DifferentAgents: Verify independent hooks per agent
- HookPersistence: Verify hooks survive beads instance recreation
- StatusTransitions: Test open -> hooked -> open -> hooked -> closed

Also fix missing json import in install_integration_test.go.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/install_integration_test.go

@@ -3,6 +3,7 @@
 package cmd
 
 import (
+	"encoding/json"
 	"os"
 	"os/exec"
 	"path/filepath"