feat(orphans): add --aggressive flag for tmux-verified orphan detection

The existing PPID=1 detection misses orphaned Claude processes that get
reparented to something other than init/launchd. The new --aggressive
flag cross-references Claude processes against active tmux sessions to
find ALL orphans not in any gt-* or hq-* session.

Testing shows this catches ~3x more orphans (117 vs 39 in one sample).

Usage:
  gt orphans procs --aggressive       # List ALL orphans
  gt orphans procs kill --aggressive  # Kill ALL orphans

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

internal/util/orphan_windows.go

@@ -18,6 +18,23 @@ type CleanupResult struct {
 	Error   error
 }
 
+// ZombieProcess represents a claude process not in any active tmux session.
+// On Windows, zombie cleanup is not supported, so this is a stub definition.
+type ZombieProcess struct {
+	PID int
+	Cmd string
+	Age int    // Age in seconds
+	TTY string // TTY column from ps
+}
+
+// ZombieCleanupResult describes what happened to a zombie process.
+// On Windows, cleanup is a no-op.
+type ZombieCleanupResult struct {
+	Process ZombieProcess
+	Signal  string // "SIGTERM", "SIGKILL", or "UNKILLABLE"
+	Error   error
+}
+
 // FindOrphanedClaudeProcesses is a Windows stub.
 func FindOrphanedClaudeProcesses() ([]OrphanedProcess, error) {
 	return nil, nil
@@ -27,3 +44,13 @@ func FindOrphanedClaudeProcesses() ([]OrphanedProcess, error) {
 func CleanupOrphanedClaudeProcesses() ([]CleanupResult, error) {
 	return nil, nil
 }
+
+// FindZombieClaudeProcesses is a Windows stub.
+func FindZombieClaudeProcesses() ([]ZombieProcess, error) {
+	return nil, nil
+}
+
+// CleanupZombieClaudeProcesses is a Windows stub.
+func CleanupZombieClaudeProcesses() ([]ZombieCleanupResult, error) {
+	return nil, nil
+}