feat(session): Include session ID in PropulsionNudge for /resume picker

PropulsionNudgeForRole now accepts a workDir parameter and reads
session ID from .runtime/session_id to append [session:xxx] to the
nudge message. This enables Claude Code's /resume picker to discover
Gas Town sessions.

(gt-u49zh)

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

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

internal/session/names_test.go

@@ -1,6 +1,11 @@
 package session
 
-import "testing"
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
 
 func TestMayorSessionName(t *testing.T) {
 	want := "gt-mayor"
@@ -102,3 +107,63 @@ func TestPrefix(t *testing.T) {
 		t.Errorf("Prefix = %q, want %q", Prefix, want)
 	}
 }
+
+func TestPropulsionNudgeForRole_WithSessionID(t *testing.T) {
+	// Create temp directory with session_id file
+	tmpDir := t.TempDir()
+	runtimeDir := filepath.Join(tmpDir, ".runtime")
+	if err := os.MkdirAll(runtimeDir, 0755); err != nil {
+		t.Fatalf("creating runtime dir: %v", err)
+	}
+
+	sessionID := "test-session-abc123"
+	if err := os.WriteFile(filepath.Join(runtimeDir, "session_id"), []byte(sessionID), 0644); err != nil {
+		t.Fatalf("writing session_id: %v", err)
+	}
+
+	// Test that session ID is appended
+	msg := PropulsionNudgeForRole("mayor", tmpDir)
+	if !strings.Contains(msg, "[session:test-session-abc123]") {
+		t.Errorf("PropulsionNudgeForRole(mayor, tmpDir) = %q, should contain [session:test-session-abc123]", msg)
+	}
+}
+
+func TestPropulsionNudgeForRole_WithoutSessionID(t *testing.T) {
+	// Use nonexistent directory
+	msg := PropulsionNudgeForRole("mayor", "/nonexistent-dir-12345")
+	if strings.Contains(msg, "[session:") {
+		t.Errorf("PropulsionNudgeForRole(mayor, /nonexistent) = %q, should NOT contain session ID", msg)
+	}
+}
+
+func TestPropulsionNudgeForRole_EmptyWorkDir(t *testing.T) {
+	// Empty workDir should not crash and should not include session ID
+	msg := PropulsionNudgeForRole("mayor", "")
+	if strings.Contains(msg, "[session:") {
+		t.Errorf("PropulsionNudgeForRole(mayor, \"\") = %q, should NOT contain session ID", msg)
+	}
+}
+
+func TestPropulsionNudgeForRole_AllRoles(t *testing.T) {
+	tests := []struct {
+		role     string
+		contains string
+	}{
+		{"polecat", "gt hook"},
+		{"crew", "gt hook"},
+		{"witness", "gt prime"},
+		{"refinery", "gt prime"},
+		{"deacon", "gt prime"},
+		{"mayor", "gt prime"},
+		{"unknown", "gt hook"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.role, func(t *testing.T) {
+			msg := PropulsionNudgeForRole(tt.role, "")
+			if !strings.Contains(msg, tt.contains) {
+				t.Errorf("PropulsionNudgeForRole(%q, \"\") = %q, should contain %q", tt.role, msg, tt.contains)
+			}
+		})
+	}
+}