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.go

@@ -197,6 +197,30 @@ Example:
 	RunE: runMoleculeAttachment,
 }
 
+var moleculeAttachFromMailCmd = &cobra.Command{
+	Use:   "attach-from-mail <mail-id>",
+	Short: "Attach a molecule from a mail message",
+	Long: `Attach a molecule to the current agent's hook from a mail message.
+
+This command reads a mail message, extracts the molecule ID from the body,
+and attaches it to the agent's pinned bead (hook).
+
+The mail body should contain an "attached_molecule:" field with the molecule ID.
+
+Usage: gt mol attach-from-mail <mail-id>
+
+Behavior:
+1. Read mail body for attached_molecule field
+2. Attach molecule to agent's hook
+3. Mark mail as read
+4. Return control for execution
+
+Example:
+  gt mol attach-from-mail msg-abc123`,
+	Args: cobra.ExactArgs(1),
+	RunE: runMoleculeAttachFromMail,
+}
+
 var moleculeStatusCmd = &cobra.Command{
 	Use:   "status [target]",
 	Short: "Show what's on an agent's hook",
@@ -350,6 +374,7 @@ func init() {
 	moleculeCmd.AddCommand(moleculeAttachCmd)
 	moleculeCmd.AddCommand(moleculeDetachCmd)
 	moleculeCmd.AddCommand(moleculeAttachmentCmd)
+	moleculeCmd.AddCommand(moleculeAttachFromMailCmd)
 
 	rootCmd.AddCommand(moleculeCmd)
 }