refactor(prime): extract findHookedBead to eliminate duplication

detectSessionState() and checkSlungWork() both contained identical
logic for finding hooked/in_progress beads assigned to an agent.
Extracted this into findHookedBead() helper function.

Also includes priming subsystem improvements from mayor:
- Add --dry-run flag for testing without side effects
- Add --state flag to output detected state only
- Add --explain flag to show why sections are included
- Add missing filepath import to beads.go

Fixes: bd-hvwnb

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

internal/beads/beads.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"strings"
 
 	"github.com/steveyegge/gastown/internal/runtime"