feat(gmail): add flattened headers to gmail get JSON output

Add a headers map with common email headers (from, to, cc, bcc, subject,
date) to the JSON output of `gog gmail get`. This makes header extraction
much simpler:

  # Before (error-prone due to jq operator precedence)
  jq '.message.payload.headers[] | select(.name == "To") | .value'

  # After
  jq '.headers.to'

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

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

Author: salmonumbrella

internal/cmd/gmail_get.go

@@ -64,7 +64,20 @@ func (c *GmailGetCmd) Run(ctx context.Context, flags *RootFlags) error {
 	}
 
 	if outfmt.IsJSON(ctx) {
-		return outfmt.WriteJSON(os.Stdout, map[string]any{"message": msg})
+		// Include a flattened headers map for easier querying
+		// (e.g., jq '.headers.to' instead of complex nested queries)
+		headers := map[string]string{
+			"from":    headerValue(msg.Payload, "From"),
+			"to":      headerValue(msg.Payload, "To"),
+			"cc":      headerValue(msg.Payload, "Cc"),
+			"bcc":     headerValue(msg.Payload, "Bcc"),
+			"subject": headerValue(msg.Payload, "Subject"),
+			"date":    headerValue(msg.Payload, "Date"),
+		}
+		return outfmt.WriteJSON(os.Stdout, map[string]any{
+			"message": msg,
+			"headers": headers,
+		})
 	}
 
 	u.Out().Printf("id\t%s", msg.Id)