feat: add gt mol attach-from-mail command (gt-h6eq.7)

Allows agents to self-pin work from mail messages. The command:
1. Reads a mail message by ID
2. Extracts molecule ID from the body (attached_molecule:, molecule_id:, etc.)
3. Attaches the molecule to the agent's pinned bead (hook)
4. Marks the mail as read

Includes unit tests for the molecule ID extraction logic.

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

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

internal/cmd/molecule_attach_from_mail_test.go

@@ -0,0 +1,80 @@
+package cmd
+
+import "testing"
+
+func TestExtractMoleculeIDFromMail(t *testing.T) {
+	tests := []struct {
+		name     string
+		body     string
+		expected string
+	}{
+		{
+			name:     "attached_molecule field",
+			body:     "Hello agent,\n\nattached_molecule: gt-abc123\n\nPlease work on this.",
+			expected: "gt-abc123",
+		},
+		{
+			name:     "molecule_id field",
+			body:     "Work assignment:\nmolecule_id: mol-xyz789",
+			expected: "mol-xyz789",
+		},
+		{
+			name:     "molecule field",
+			body:     "molecule: gt-task-42",
+			expected: "gt-task-42",
+		},
+		{
+			name:     "mol field",
+			body:     "Quick task:\nmol: gt-quick\nDo this now.",
+			expected: "gt-quick",
+		},
+		{
+			name:     "no molecule field",
+			body:     "This is just a regular message without any molecule.",
+			expected: "",
+		},
+		{
+			name:     "empty body",
+			body:     "",
+			expected: "",
+		},
+		{
+			name:     "molecule with extra whitespace",
+			body:     "attached_molecule:   gt-whitespace  \n\nmore text",
+			expected: "gt-whitespace",
+		},
+		{
+			name:     "multiple fields - first wins",
+			body:     "attached_molecule: first\nmolecule: second",
+			expected: "first",
+		},
+		{
+			name:     "case insensitive line matching",
+			body:     "Attached_Molecule: gt-case",
+			expected: "gt-case",
+		},
+		{
+			name:     "molecule in multiline context",
+			body: `Subject: Work Assignment
+
+This is your next task.
+
+attached_molecule: gt-multiline
+
+Please complete by EOD.
+
+Thanks,
+Mayor`,
+			expected: "gt-multiline",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := extractMoleculeIDFromMail(tt.body)
+			if result != tt.expected {
+				t.Errorf("extractMoleculeIDFromMail(%q) = %q, want %q", tt.body, result, tt.expected)
+			}
+		})
+	}
+}