Add StartupNudge function for unified session metadata (gt-bgfqy)

Creates internal/session/startup.go with StartupNudgeConfig struct
and StartupNudge function. Format becomes session title in /resume picker:
[GAS TOWN] recipient <- sender • timestamp • topic[:mol-id]

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

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

internal/session/startup_test.go

@@ -0,0 +1,103 @@
+package session
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestFormatStartupNudge(t *testing.T) {
+	tests := []struct {
+		name     string
+		cfg      StartupNudgeConfig
+		wantSub  []string // substrings that must appear
+		wantNot  []string // substrings that must NOT appear
+	}{
+		{
+			name: "assigned with mol-id",
+			cfg: StartupNudgeConfig{
+				Recipient: "gastown/crew/gus",
+				Sender:    "deacon",
+				Topic:     "assigned",
+				MolID:     "gt-abc12",
+			},
+			wantSub: []string{
+				"[GAS TOWN]",
+				"gastown/crew/gus",
+				"<- deacon",
+				"assigned:gt-abc12",
+			},
+		},
+		{
+			name: "cold-start no mol-id",
+			cfg: StartupNudgeConfig{
+				Recipient: "deacon",
+				Sender:    "mayor",
+				Topic:     "cold-start",
+			},
+			wantSub: []string{
+				"[GAS TOWN]",
+				"deacon",
+				"<- mayor",
+				"cold-start",
+			},
+			// No wantNot - timestamp contains ":"
+		},
+		{
+			name: "handoff self",
+			cfg: StartupNudgeConfig{
+				Recipient: "gastown/witness",
+				Sender:    "self",
+				Topic:     "handoff",
+			},
+			wantSub: []string{
+				"[GAS TOWN]",
+				"gastown/witness",
+				"<- self",
+				"handoff",
+			},
+		},
+		{
+			name: "mol-id only",
+			cfg: StartupNudgeConfig{
+				Recipient: "gastown/polecats/Toast",
+				Sender:    "witness",
+				MolID:     "gt-xyz99",
+			},
+			wantSub: []string{
+				"[GAS TOWN]",
+				"gastown/polecats/Toast",
+				"<- witness",
+				"gt-xyz99",
+			},
+		},
+		{
+			name: "empty topic defaults to ready",
+			cfg: StartupNudgeConfig{
+				Recipient: "deacon",
+				Sender:    "mayor",
+			},
+			wantSub: []string{
+				"[GAS TOWN]",
+				"ready",
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := FormatStartupNudge(tt.cfg)
+
+			for _, sub := range tt.wantSub {
+				if !strings.Contains(got, sub) {
+					t.Errorf("FormatStartupNudge() = %q, want to contain %q", got, sub)
+				}
+			}
+
+			for _, sub := range tt.wantNot {
+				if strings.Contains(got, sub) {
+					t.Errorf("FormatStartupNudge() = %q, should NOT contain %q", got, sub)
+				}
+			}
+		})
+	}
+}